sock: Introduce xmpp_sock_t abstraction

libstrophe uses non-blocking sockets and the connect() syscall may
return before a TCP connection is established. This doesn't allow
to catch all possible errors synchronously and some of the errors
are handled in the event handler.

In a scenario with multiple SRV records and/or multiple IP addresses
resolution, we need to repeat connection attempt on a failure.

xmpp_sock_t resolves the above problem. It keeps resolved records and
addresses to repeat connect attempt asynchronously.

src/common.h

@@ -198,6 +198,7 @@ struct _xmpp_conn_t {
     int error;
     xmpp_stream_error_t *stream_error;
 
+    xmpp_sock_t *xsock;
     sock_t sock;
     int ka_timeout;  /* TCP keepalive timeout */
     int ka_interval; /* TCP keepalive interval */