- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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

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