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/resolver.h

@@ -36,6 +36,12 @@ typedef struct resolver_srv_rr_struc {
 void resolver_initialize(void);
 void resolver_shutdown(void);
 
+resolver_srv_rr_t *resolver_srv_rr_new(xmpp_ctx_t *ctx,
+                                       const char *host,
+                                       unsigned short port,
+                                       unsigned short prio,
+                                       unsigned short weight);
+
 /** Perform lookup for RFC1035 message format.
  *  This function allocates all elements.
  *