This implementation uses abstract sockets for process notifications,
similar to systemd's notify sockets, but notifiers use a PID-specific
name from a well-known namespace and listeners are assumed to be
ephemeral.
Update swift-reload to use these instead of polling child processes to
determine when a server reload has completed. Bonus: it also acts as a
non-blocking lock to prevent two swift-reload commands from reloading a
process at the same time.
Closes-Bug: #2098405
Related-Change: Ib2dd9513d3bb7c7686e6fa35485317bbad915876
Change-Id: I5f36aba583650bddddff5e55ac557302d023ea1b
313fab5f5f