Make 'gs-netcat [cmd]' work like in ssh

[SkyperTHC]

thanks L.

Can be done as we already have an inband signaling protocol (pkt) and thus can submit command line to remote or indicate that
this is a non-tty cmd-exec shell. Will only work for versions >= 1.4.34 but without breaking backwards compatibility (can use protocol minor version indicator and flag to gsrnd to indicate command-exec shell).

Shall we use /bin/sh -c or $SHELL -c? Take a look at ssh and do it how they do it (i guess $SHELL as /bin/sh is for losers).