manager: add socket for receiving handoff timestamps from forked children

manager: add socket for receiving handoff timestamps from forked children

This adds an AF_UNIX socket pair to the manager that we can collect
handoff timestamp messages on.

The idea is that forked off children send a datagram with a timestamp
and we use its sender PID to match it against the right forked off
process.

This part only implements the receiving side: a socket is created, and
listened on. Received datagrams are parsed, verified and then dispatched
to the interested units.