Add TCP connection support, proxy server

Trivial TCP listener which proxies data from TCP (only one
connection at a time, currently implemented trivially with
blocking...) to a USB NXT.

Plus matching TCP-NXT support on the client side.

Mainly useful for replacing the NXT's too slow bluetooth support with
TCP, assuming a suitable server for the TCP proxy is connected through
USB.
Tweak some types in nxt-set-output-state, nxt-message-read