HTTP: Support Apache-based servers with Negotiate

Disambiguate between general network problems and HTTP problems in error
codes.

Download poxygit, a debugging git server, and clone from it using NTLM,
both IIS-style (with connection affinity) and Apache-style ("broken",
requiring constant reauthentication).

When tracing is disabled, don't let `git_trace__level` return a void,
since that can't be compared against.

When we're authenticating with a connection-based authentication scheme
(NTLM, Negotiate), we need to make sure that we're still connected
between the initial GET where we did the authentication and the POST
that we're about to send.  Our keep-alive session may have not kept
alive, but more likely, some servers do not authenticate the entire
keep-alive connection and may have "forgotten" that we were
authenticated, namely Apache and nginx.

Send a "probe" packet, that is an HTTP POST request to the upload-pack
or receive-pack endpoint, that consists of an empty git pkt ("0000").
If we're authenticated, we'll get a 200 back.  If we're not, we'll get a
401 back, and then we'll resend that probe packet with the first step of
our authentication (asking to start authentication with the given
scheme).  We expect _yet another_ 401 back, with the authentication
challenge.

Finally, we will send our authentication response with the actual POST
data.  This will allow us to authenticate without draining the POST data
in the initial request that gets us a 401.

Untangle the notion of the http transport from the actual http
implementation.  The http transport now uses the httpclient.

Allow users to opt-in to expect/continue handling when sending a POST
and we're authenticated with a "connection-based" authentication
mechanism like NTLM or Negotiate.

If the response is a 100, return to the caller (to allow them to post
their body).  If the response is *not* a 100, buffer the response for
the caller.

HTTP expect/continue is generally safe, but some legacy servers
have not implemented it correctly.  Require it to be opt-in.

Fully support HTTP proxies, in particular CONNECT proxies, that allow us
to speak TLS through a proxy.

Detect responses that are sent with Transfer-Encoding: chunked, and
record that information so that we can consume the entire message body.

Store the last-seen credential challenges (eg, all the
'WWW-Authenticate' headers in a response message).  Given some
credentials, find the best (first) challenge whose mechanism supports
these credentials.  (eg, 'Basic' supports username/password credentials,
'Negotiate' supports default credentials).

Set up an authentication context for this mechanism and these
credentials.  Continue exchanging challenge/responses until we're
authenticated.

Introduce a function to format the path and query string for a URL,
suitable for creating an HTTP request.

When sending a new request, ensure that we got the entirety of the
response body.  Our caller may have decided that they were done reading.
If we were not at the end of the message, this means that we need to
tear down the connection and cannot do keep-alive.

However, if the caller read all of the message, but we still have a
final end-of-response chunk signifier (ie, "0\r\n\r\n") on the socket,
then we should consider that the response was successfully copmleted.

If we're asked to send a new request, try to read from the socket, just
to clear out that end-of-chunk message, marking ourselves as
disconnected on any errors.

The CLAR_TRACE_LEVEL environment variable was supported when building
with GIT_TRACE.  Now we always build with GIT_TRACE, but that variable
is not provided to tests.  Simply support clar tracing always.

Teach httpclient how to support chunking when POSTing request bodies.

Introduce a new http client implementation that can GET and POST to
remote URLs.

Consumers can use `git_http_client_init` to create a new client,
`git_http_client_send_request` to send a request to the remote server
and `git_http_client_read_response` to read the response.

The http client implementation will perform the I/O with the remote
server (http or https) but does not understand the git smart transfer
protocol.  This allows us to split the concerns of the http subtransport
from the actual http implementation.

Allow users to consume a buffer by the number of bytes, not just to an
ending pointer.

Provide a mechanism to add a path and query string to an existing url
so that we can easily append `/info/refs?...` type url segments to a url
given to us by a user.

Move the redirect handling into `git_net_url` for consistency.

(Also, mark all the declarations as extern.)

Clarify what it means to not send a length; this allows us to refactor
requests further.

We can't get a kerberos TGT easily on Windows; skip the negotiate tests.

This conditional was backwards.  We should instead test that clone
returns 4321, not that 4321 returns clone.

Disposal pattern; dispose on completion, allowing us to retry
authentication, which may happen on web servers that close
connection-based authenticated sessions (NTLM/SPNEGO) unexpectedly.

When USE_GSSAPI=OFF, still show information about what SPNEGO is, even
though it's disabled.  This is for parity with other disabled features
like SSH and debugpool that still show details about what is disabled.

The GSS_ERROR(x) macro may expand to `(x & value)` on some
implementations, instead of `((x) & value)`.  This is the case on macOS,
which means that if we attempt to wrap an expression in that macro, like
`a = b`, then that would expand to `(a = b & value)`.

Since `&` has a higher precedence, this is not at all what we want, and
will set our result code to an incorrect value.  Evaluate the expression
then test it with `GSS_ERROR` independently to avoid this.

Attempt to obtain a Kerberos ticket from LIBGIT2.ORG and then clone the
Negotiate-protected site at test.libgit2.org with that ticket.

internal types: change enums from `type_t` to `_t`

libgit2 does not use `type_t` suffixes as it's redundant; thus, rename
`git_http_authtype_t` to `git_http_auth_t` for consistency.

libgit2 does not use `type_t` suffixes as it's redundant; thus, rename
`git_iterator_type_t` to `git_iterator_t` for consistency.