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.

doc/reference.html

@@ -145,6 +145,8 @@ Support, Manual">
 <blockquote>
 <a href="socket.html#bind">bind</a>,
 <a href="socket.html#connect">connect</a>,
+<a href="socket.html#connect">connect4</a>,
+<a href="socket.html#connect">connect6</a>,
 <a href="socket.html#debug">_DEBUG</a>,
 <a href="dns.html#dns">dns</a>,
 <a href="socket.html#gettime">gettime</a>,