vke(4) - Dynamic mbuf cluster allocation.

* Calculate the number of mbuf clusters per ring buffer
  based on the total amount available in the system.

* Up to 50% of the total mbuf clusters is shared among
  all the vke devices configured in a vkernel.
varsym(1) - Fix return codes.

* varsym was always returning 0 (success) which was incorrect.
* Return codes are now as indicated in the man page.

Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/2517>
Reported-by: Dmitry Razumovsky
ufs.5 - Add missing MLINKS
kernel - Do not clean VM pages on fsync() for tmpfs

* tmpfs fsync()s are basically NOPs, but the kernel had some
  code to clean the VM object (flush to backing store) on
  fsync, which would actually cause pages to go out to swap
  unnecessarily.

* Detect the appropriate flag and turn this off.  Only effects
  tmpfs.

* Improves poudriere and other tmpfs-related use cases.