add support for identityagent ssh_config option

This commit adds an `agent_socket` field to the session options
and connects the config parser to that option.

`SSH_OPTIONS_IDENTITY_AGENT` is added to allow applications to
set this option for themselves.

agent.c is updated to take the `agent_socket` value in preference
to the `SSH_AUTH_SOCK` environment variable.

Signed-off-by: Wez Furlong <wez@fb.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/libssh.h

@@ -407,6 +407,7 @@ enum ssh_options_e {
   SSH_OPTIONS_REKEY_DATA,
   SSH_OPTIONS_REKEY_TIME,
   SSH_OPTIONS_RSA_MIN_SIZE,
+  SSH_OPTIONS_IDENTITY_AGENT,
 };
 
 enum {