process-util: Use pidref_wait_for_terminate_and_check in pidref_safe_fork()

process-util: Use pidref_wait_for_terminate_and_check in pidref_safe_fork()

Note that we still have to block SIGCHLD so that
we can be certain the process is not reaped before
we get the pidfd to it. safe_fork() and friends are
used in libsystemd where we don't control how the
SIGCHLD signal is configured. Specifically, kernel
autoreaping could be enabled which is why we have
to block SIGCHLD until we get the pidfd so that the
kernel cannot autoreap the process before we get
the pidfd.