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.

Increase the permissible replay count; with multiple-step authentication
schemes (NTLM, Negotiate), proxy authentication and redirects, we need
to be mindful of the number of steps it takes to get connected.

7 seems high but can be exhausted quickly with just a single authentication
failure over a redirected multi-state authentication pipeline.

Put a limit on the number of authentication replays in the HTTP
transport.  Standardize on 7 replays for authentication or redirects,
which matches the behavior of the WinHTTP transport.

The winhttp and posix http each need to add the user-agent to their
requests.  Standardize on a single function to include this so that we
do not get the version numbers we're sending out of sync.

Assemble the complete user agent in `git_http__user_agent`, returning
assembled strings.

Co-authored-by: Patrick Steinhardt <ps@pks.im>