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

Test *at(2) calls with capability-mode lookup.

This commit adds regression testing for openat(), fstatat(), etc. with
capability scoping ("strict relative" lookup), which applies:
- in capability mode
- when performing any *at() lookup relative to a capability

These tests will fail until the *at() code is committed; on my local
instance, with the *at() changes, they all pass.

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

Flesh out the cap_test regression test.

Add more regression testing, some of which is expected to fail until we
commit more kernel implementation.

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

Regression tests for Capsicum capability mode.

Ensure that system calls that access global namespaces, e.g. open(2), are not permitted, and that whitelisted sysctls like kern.osreldate are.

Approved by: rwatson
Sponsored by: Google, Inc.