rpcbind: Fix race in signal termination

If a signal was delivered while the main thread was not in poll(2) and after
check was performed, we could reenter poll and never detect termination. Fix
this with the pipefd trick. (This race was introduced very recently, in
r327482.)

PR: 224503
Reported by: kib
Reviewed by: kib, markj
Sponsored by: Dell EMC Isilon