Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Fixed missing or broken library dependencies.

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.

Add sigqueue test code.