channel_forward, and moved it to a separate file. It's not really
tcpip-forwarding specific, and can be reused for x11-forwarding.

Rev: src/channel_forward.c:1.1
Rev: src/channel_forward.h:1.1
Rev: src/tcpforward.c:1.62
Rev: src/tcpforward.h:1.17
Rev: src/tcpforward_commands.c:1.33