Job control improvements

[caseman]

Draft fix for https://github.com/emersion/mrsh/issues/174

• Fixes SIGSTOP/SIGCONT signal handling
• removes intermediate fork in run_command_list_array for background jobs so that actual child state can be tracked properly
• improves reliability of job status updates and notifications

I included a jobs.sh test script, but it isn't incorporated into the test target because it causes it hang. Running it separately it works fine though and exits cleanly. Also not sure if these tests are only for parity, if so then it needs more attention since I didn't attempt to run it in other shells yet.

See also XXX comments inline. In particular the one about the general fork abstraction. I feel like forking a job process could be generalized since it happens in several places.

[caseman]

Superceded by: https://github.com/emersion/mrsh/pull/176