add the listen1 command.

To use this command, e.g.
listen1 tcp!*!23 /bin/ash

Listens on port 23. The best thing we've found to talk to
it is netcat.

To make it available to localhost at 5555
qemu ... \
-net user,hostfwd=tcp::5555-:23 \

to talk to it:
netcat localhost 5555

Signed-off-by: Ronald G. Minnich <rminnich@google.com>

Signed-off-by: Ronald G. Minnich <rminnich@google.com>