Adjust some code in mud_client.

Lua has no Non-Preemptive Multithreading(maybe you can build it
yourself in http://www.lua.org/pil/9.4.html). Multi-thread coding
is just a little uncomfortbale.

The connection may be ended unexpectedly when I play for a while.
Maybe the server can detect and hang up strange connection?
Well, maybe I should focus on Lua's advantage, glue languange(also
limitation), put these socket things to C++.