Agent forwarding implementation (#752)

This PR contains a series of patches that date back many years and I
believe were discussed on the mailing list, but never merged. We have
been using these in our local copy of libssh2 without issue since 2015,
if not earlier. I believe this is the full set of changes, as we tried
to use comments to mark where our copy of libssh2 differs from the
canonical version.

This also contains changes I made earlier this year, but which were not
discussed on the mailing list, to support certificates and FIDO2 keys
with agent forwarding.

Note that this is not a complete implementation of agent forwarding, as
that is outside the scope of libssh2. Clients still need to provide
their own implementation that parses ssh-agent methods after calling
libssh2_channel_read() and calls the appropriate callback messages in
libssh2. See the man page changes in this PR for more details.

Integration-patches-by: Viktor Szakats

* prefer size_t
* prefer unsigned int over u_int in public function
* add const
* docs, indent, checksrc, debug call, compiler warning fixes

docs/libssh2_agent_userauth.3

@@ -30,3 +30,4 @@ Added in libssh2 1.2
 .SH SEE ALSO
 .BR libssh2_agent_init(3)
 .BR libssh2_agent_get_identity(3)
+.BR libssh2_agent_sign(3)