Add OpenSSH support

Handle custom paths for OpenSSH.

This helped when troubleshooting issues running the `ci/test.sh` script
locally.

Callers can specify the ssh command to invoke using `core.sshcommand` or
the `GIT_SSH` environment variable. This is useful for specifying
alternate configuration, and is particularly useful for our testing
environment.

Provide both cmdline-style handling (passing it to the shell on POSIX,
or directly to CreateProcess on win32) and execv style (passing it
directly to execv on POSIX, and mangling it into a single command-line
on win32).

Don't capture stderr, optimize for the CLI case.

Provide more user-friendly error messages in smart protocol negotiation
failures.

Provide a mechanism for callers to read from stderr.

Suppress SIGPIPEs during writes to our piped process. On single-threaded
applications, this is as simple as ignoring the signal. But since this
is process-wide, on multi-threaded applications, we need to use some
cumbersome `pthread_sigmask` manipulation.

Thanks to https://www.doof.me.uk/2020/09/23/sigpipe-and-how-to-ignore-it/
and http://www.microhowto.info:80/howto/ignore_sigpipe_without_affecting_other_threads_in_a_process.html

Instead of "early EOF", provide information on _when_ we're seeing the
EOF for debugging.

A transport may want to validate that it's in a sane state; when
flushing on close, don't assume that we're doing an upload-pack; send
the correct direction.

Now that we (may) exec a child process to do ssh, we don't want valgrind
reporting on that. Suppress children in valgrind runs.

There are no custom callbacks for OpenSSH; don't test them.

We can't reliably detect SIGPIPE on close because of platform
differences. Track `pid` and send `SIGTERM` to a function and ensure
that we can detect it.

We can now use the `git_process` class to invoke OpenSSH and use it as
an SSH transport. This may be preferred over libssh2 for a variety of
callers.

We may want to support SSH but with a different provider that is not
libssh2. Add GIT_SSH to indicate that we have some inbuilt SSH support
and GIT_SSH_LIBSSH2 to indicate that support is via libssh2. This is
similar to how we support GIT_HTTPS and GIT_OPENSSL, for example.

ci: update to macOS 12

ci: retry flaky online tests

cmake: fix openssl build on win32

since f15c8ac7 libgit unconditionally depends on secur32 on Windows
but only added it in cmake for the winhttp and schannel variants.
In case libgit is built against openssl it would fail to link.

This moves secur32 out of the https backend selection code into
the global win32 condition (and while at it also adds ws2_32 to the .pc file)

cli: add a `git config` command

config: provide an "origin" for config entries

README.md: Fix link to conan packages

Revert "CMake: Search for ssh2 instead of libssh2."

proxy: Return an error for invalid proxy URLs instead of crashing.

Test proxies specified by both host:port format in configuration
options, environment variables, and `http.proxy` configuration.

The common format for specifying proxy URLs is just 'host:port'. Handle
the common case.

Introduce a url parser that defaults to treating poorly specified URLs
as http URLs. For example: `localhost:8080` is treated as
`http://localhost:8080/` by the http-biased url parsing, instead of a
URL with a scheme `localhost` and a path of `8080`..