net-mgmt/unifi5: Clean up child processes

Unifi starts up its own mongodb child process automatically. It does
this regardless of any existing mongodb process running on the system.
If you attempt to restart unifi without stopping its mongodb process it
will attempt to start mongodb processes for eternity, filling your logs.
This situation is regularly encountered by users updating to the newest
release.

There is no workaround but to signal any remaining processes owned by
the unifi user at shutdown. The included change takes note of any child
PIDs before starting shutdown and signals them after the unifi process
has been stopped successfully.

Reported by: dvl