config: Preserve the seen array among invocations

This follows the OpenSSH behavior of parsing subseqent configuration
files, while applying only the first option.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/session.h

@@ -220,6 +220,7 @@ struct ssh_session_struct {
         int flags;
         int nodelay;
         bool config_processed;
+        uint8_t *options_seen;
     } opts;
     /* counters */
     ssh_counter socket_counter;