timeout: handle implicitly created threads

timeout: handle implicitly created threads

On some systems like glibc on GNU/kFreeBSD, a thread is
implicitly created when timer_settime() is used.
This breaks our scheme to ignore signals we've
sent ourselves.

* src/timeout.c (send_sig): Change the scheme used to
ignore signals we've sent ourselves, to a more robust
but perhaps limited scheme of ignoring all signals of
a certain type after we've sent that signal to the job.
* NEWS: Mention the change in behavior.