process-util: add pidref_safe_fork() helper

process-util: add pidref_safe_fork() helper

This combines safe_fork() with pidref_set_pid().

Eventually we really should switch this to use CLONE_PIDFD, but as that
is not wrapped by glibc yet, it's hard. But this is not crucial anyway,
as a child we just forked off can always safely be referenced also by
PID, given the reaping is under our own control.

A simple test case is added in a follow-up commit.