Clarify lifetime of child(..) function

Ensure child exits when complete as it's always run in a forked
process.

Add a missing break statement in :pselect_sigmask when calling
child(..) for clarity and to avoid weird domino effects if the
child process somehow does something it's not supposed to do
with the logfiles, file descriptors, etc

MFC after: 1 week
Reported by: Coverity
CID: 1223369, 1223370, 1300301