Support for direct-streamlocal@openssh.com UNIX socket connection (#945)

This patch allow to use direct-streamlocal service from OpenSSH 6.7,
that allows UNIX socket connections.

Mods:
- delete unrelated condition:
  Ref: https://github.com/libssh2/libssh2/pull/216#discussion_r374748111
- rebase on master, whitespace updates.

Patch-by: @gjalves Gustavo Junior Alves

Closes #216
Closes #632
Closes #945

docs/libssh2_channel_direct_streamlocal_ex.3

@@ -0,0 +1,33 @@
+.TH libssh2_channel_direct_streamlocal_ex 3 "10 Apr 2023" "libssh2 1.11.0" "libssh2 manual"
+.SH NAME
+libssh2_channel_direct_streamlocal_ex - Tunnel a UNIX socket connection through an SSH session
+.SH SYNOPSIS
+#include <libssh2.h>
+
+LIBSSH2_CHANNEL *
+libssh2_channel_direct_streamlocal_ex(LIBSSH2_SESSION *session, const char *socket_path, const char *shost, int sport);
+
+LIBSSH2_CHANNEL *
+libssh2_channel_direct_streamlocal(LIBSSH2_SESSION *session, const char *socket_path, const char *shost, int sport);
+
+.SH DESCRIPTION
+\fIsession\fP - Session instance as returned by
+.BR libssh2_session_init_ex(3)
+
+\fIsocket_path\fP - UNIX socket to connect to using the SSH host as a proxy.
+
+\fIshost\fP - Host to tell the SSH server the connection originated on.
+
+\fIsport\fP - Port to tell the SSH server the connection originated from.
+
+Tunnel a UNIX socket connection through the SSH transport via the remote host to
+a third party. Communication from the client to the SSH server remains
+encrypted, communication from the server to the 3rd party host travels
+in cleartext.
+
+.SH RETURN VALUE
+Pointer to a newly allocated LIBSSH2_CHANNEL instance, or NULL on errors.
+.SH ERRORS
+\fILIBSSH2_ERROR_ALLOC\fP -  An internal memory allocation call failed.
+.SH SEE ALSO
+.BR libssh2_session_init_ex(3)