BUG/MINOR: systemd: ensure we don't miss signals

BUG/MINOR: systemd: ensure we don't miss signals

There's a race condition in the systemd wrapper. People who restart it
too fast report old processes remaining there. Here if the signal is
caught before entering the "while" loop we block indefinitely on the
wait() call. Let's check the caught signal before calling wait(). It
doesn't completely close the race, a window still exists between the
test of the flag and the call to wait(), but it's much smaller. A
safer solution could involve pselect() to wait for the signal
delivery.