process-util: make pidref_safe_fork_full() work with FORK_WAIT

process-util: make pidref_safe_fork_full() work with FORK_WAIT

(This is useful for the test case added in the next commit, where it's
kinda nice being able to use pidref_safe_fork_full() and acquiring a
pidref of the child in the child in one go. There's no other value in
this than a bit of synctactic sugar for that test. But otoh thre's no
good reason to prohibit FORK_WAIT use like this, hence either way, this
commit should be a good thing.)