CONTRIB: tcploop: implement fork()

CONTRIB: tcploop: implement fork()

Fork is a very convenient way to deal with independant yet properly
timed connections. It's particularly useful here for accept(), and
ensures that any accepted FD will automatically be released. The
principle is that when we hit a fork command, the parent restarts
evaluating the actions from the beginning and the child continues
to evaluate the next actions. Listen and connect are skipped if the
connection is already established. Fork() is amazingly cheap on
Linux, 21k forked connections per second are handled on a single
core, and 38k on two cores.

For now it's not possible to have two different code paths so in order
to have both a listener and a connector, two distinct commands are
still needed.