run-command: optionally kill children on exit

When we spawn a helper process, it should generally be done
and finish_command called before we exit. However, if we
exit abnormally due to an early return or a signal, the
helper may continue to run in our absence.

In the best case, this may simply be wasted CPU cycles or a
few stray messages on a terminal. But it could also mean a
process that the user thought was aborted continues to run
to completion (e.g., a push's pack-objects helper will
complete the push, even though you killed the push process).

This patch provides infrastructure for run-command to keep
track of PIDs to be killed, and clean them on signal
reception or input, just as we do with tempfiles. PIDs can
be added in two ways:

  1. If NO_PTHREADS is defined, async helper processes are
     automatically marked. By definition this code must be
     ready to die when the parent dies, since it may be
     implemented as a thread of the parent process.

  2. If the run-command caller specifies the "clean_on_exit"
     option. This is not the default, as there are cases
     where it is OK for the child to outlive us (e.g., when
     spawning a pager).

PIDs are cleared from the kill-list automatically during
wait_or_whine, which is called from finish_command and
finish_async.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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