fork-notify: Use callback instead of argv NULL code path with return

fork-notify: Use callback instead of argv NULL code path with return

In 012d87c1fc/cc8f398202 it was made possible for fork_notify() to
return in the child but at that point all FDs were closed and the
_cleanup path from the return causes assertion failures due to invalid
FDs in notify_event_source/event, leading to a vmspawn failing to start
with a SIGABRT logged in coredump.
Instead of TAKE_PTR on a bunch of things which is fragile, rather avoid
the return and instead add an explicit callback handler and guarantee
to exit directly after it. A userdata argument is also added but not
used yet I think it's quite normal to have for a callback.