Low: libservices (sync): ensure no zombie is left behind

It could happen because this parent that is to wait for its
not-well-behaved child is knowlingly not blocking any signal (beside
SIGCHLD explicitly in case of using signalfd facility) and delivery of
such signal can interrupt waitpid, at least on paper, so protect
against this accordingly.

Speaking of SIGCHLD in plain self-pipe (not signalfd one) context, while
there's no clash with other synchronous actions, it may be the case with
asynchronous ones (or for that matter, arbitrary other fork-related
activities in the main program a library can have no idea about), and
this is exactly what could interrupt waitpid for real.