bitbake: bitbake-worker: Improve sigterm handler

bitbake: bitbake-worker: Improve sigterm handler

When processes terminate, we really want all of the child processes to
terminate too. This was not happening for worker processes which spawned their
own multiprocessing pools, leading to build hangs. This change ensures any
sigterm gets passed to the whole process group. In local tests, this resolved
some hanging process workloads I could generate. It does rely on signals
being delivered in a timely fashion and there is a multiprocessing bug we have
to work around there.

(Bitbake rev: 96f8ea07ace1379380fab2d78eb592fa40c867d4)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>