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

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

Give fstest a more unique name: pjdfstest.
It is released from time to time and used outside FreeBSD, so it is good to
have a name one can google.

Add 3436 file system regression tests in 184 files.
Almost all regression tests are based on very flexible fstest tool.
They verify correctness (POSIX conformance) of almost all file
system-related system calls.

The motivation behind this work is my ZFS port and POSIX, who doesn't
provide free test suites.

Runs on: FreeBSD/UFS, FreeBSD/ZFS, Solaris/UFS, Solaris/ZFS

To try it out:

# cd fstest
# make
# find tests/* -type d | xargs prove