Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Tweaks for sigqueue tests:
- slightly adjust code for style, sort headers.
- in sigqtest2, print received signals, to make it easy to see why test
failed.
- in sigqtest2, job_control_test(), cover a race by adding sleep after
child stopped itself to allow for SIGCHLD due to stop and exit to not
be coalesced.

MFC after: 2 weeks

Add code to test queued SIGCHLD.