net.server_epoll: Fix reporting of socket connect timeout

If the underlying TCP connection times out before the write timeout
kicks in, end up here with err="timeout", which the following code
treats as a minor issue.

Then, due to epoll apparently returning the EPOLLOUT (writable) event
too, we go on and try to write to the socket (commonly stream headers).
This fails because the socket is closed, which becomes the error
returned up the stack to the rest of Prosody.

This also trips the 'onconnect' signal, which has effects on various
things, such as the net.connect state machine. Probably undesirable
effects.

With this, we instead return "connection timeout", like server_event,
and destroy the connection handle properly. And then nothing else
happens because the connection has been destroyed.

net/server_epoll.lua

@@ -416,6 +416,8 @@ function interface:onreadable()
 		elseif err == "wantwrite" then
 			self:set(nil, true);
 			err = "timeout";
+		elseif err == "timeout" and not self._connected then
+			err = "connection timeout";
 		end
 		if partial and partial ~= "" then
 			self:onconnect();