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_sign.3

@@ -0,0 +1,52 @@
+.TH libssh2_agent_sign 3 "1 Oct 2022" "libssh2 1.11.0" "libssh2 manual"
+.SH NAME
+libssh2_agent_sign - sign data, with the help of ssh-agent
+.SH SYNOPSIS
+.nf
+#include <libssh2.h>
+
+int
+libssh2_agent_sign(LIBSSH2_AGENT *agent,
+                   struct libssh2_agent_publickey *identity,
+                   unsigned char **sig,
+                   size_t *s_len,
+                   const unsigned char *data,
+                   size_t d_len,
+                   const char *method,
+                   unsigned int method_len);
+.fi
+.SH DESCRIPTION
+\fIagent\fP - ssh-agent handle as returned by
+.BR libssh2_agent_init(3)
+
+\fIidentity\fP - Public key to authenticate with, as returned by
+.BR libssh2_agent_get_identity(3)
+
+\fIsig\fP - A pointer to a buffer in which to place the signature. The caller
+is responsible for freeing the signature with LIBSSH2_FREE.
+
+\fIs_len\fP - A pointer to the length of the sig parameter.
+
+\fIdata\fP - The data to sign.
+
+\fId_len\fP - The length of the data parameter.
+
+\fImethod\fP - A buffer indicating the signing method. This should match the
+string at the start of identity->blob.
+
+\fImethod_len\fP - The length of the method parameter.
+
+Sign data using an ssh-agent. This function can be used in a callback
+registered with libssh2_session_callback_set(3) using
+LIBSSH2_CALLBACK_AUTHAGENT_SIGN to sign an authentication challenge from a
+server. However, the client is responsible for implementing the code that calls
+this callback in response to a SSH2_AGENTC_SIGN_REQUEST message.
+.SH RETURN VALUE
+Returns 0 if succeeded, or a negative value for error.
+.SH AVAILABILITY
+Added in libssh2 1.11.0
+.SH SEE ALSO
+.BR libssh2_agent_init(3)
+.BR libssh2_agent_get_identity(3)
+.BR libssh2_agent_userauth(3)
+.BR libssh2_session_callback_set(3)