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)

Test process descriptors.

Ensure that process descriptors work as expected. We should be able to:
- pdfork(), like regular fork(), but producing a process descriptor
- pdgetpid() to convert a PD into a PID
- pdkill() to send signals to a process identified by a PD

Approved by: re (kib), mentor (rwatson)
Sponsored by: Google Inc