spawn: pi_fork_exec: restore parent sigmask in child
We continue to unblock SIGCHLD unconditionally, but also
any signals not blocked by the parent (wq_worker).
This will allow Ctrl-C (SIGINT) to stop "git clone" and allow
git-clone cleanup to be performed and other long-running
processes when pi_fork_exec supports setpgid(2). This won't
affect existing daemons on systems with signalfd(2) or
EVFILT_SIGNAL at all, since those run with signals blocked
anyways.