socket.connect now implemented in the C core

This avoid socket.lua duplicating the iteration over the results
of getaddrinfo(). Some problems with the C implementation not
initializing sockets or the luasocket family have also been fixed,
and error reporting made more robust.

src/usocket.c

@@ -447,7 +447,7 @@ const char *socket_gaistrerror(int err) {
         case EAI_SERVICE: return "service not supported for socket type";
         case EAI_SOCKTYPE: return "ai_socktype not supported";
         case EAI_SYSTEM: return strerror(errno); 
-        default: return "unknown error";
+        default: return gai_strerror(err);
     }
 }