MEDIUM: jobs: support unstoppable jobs for soft stop

MEDIUM: jobs: support unstoppable jobs for soft stop

This patch allows a process to properly quit when some jobs are still
active, this feature is handled by the unstoppable_jobs variable, which
must be atomically incremented.

During each new iteration of run_poll_loop() the break condition of the
loop is now (jobs - unstoppable_jobs) == 0.

The unique usage of this at the moment is to handle the socketpair CLI
of a the worker during the stopping of the process.  During the soft
stop, we could mark the CLI listener as an unstoppable job and still
handle new connections till every other jobs are stopped.