kernel: Fix -Wundef in a number of places.
kernel: Remove <machine/apm_segments.h>.

As far as I can see, those headers were never used for anything during
the lifetime of DragonFly.
kernel - Remove improper EBUSY error on mount

* Remove the old VMOUNT vnode flag.  This flag is no longer applicable
  to anything we do.

* Simultanious mounts onto the same target at different points in the
  topology could cause races against the VMOUNT flag due to the vnode
  being shared, resulting in an EBUSY error for a perfectly good mount
  request.

  For example, if you mount_null A to X, A to Y, and A to Z, then
  you simultaniously try to mount devfs onto X/dev, Y/dev, and Z/dev,
  some of those dev mount attempts could fail.  This was due to the
  VMOUNT flag set on the vnode.  X/dev, Y/dev, and Z/dev share the
  same physical vnode.

* Fixes poudriere weirdness when many jails are in use.
kernel - Remove improper EBUSY error on mount (2)

* Fix lock order reversal in checkdirs() exposed by the removal
  of the VMOUNT flag.
world - Fix world build

* Fix world build for VMOUNT flag removal.
vkernel64 - Remove some dead code (JGV).
kernel - Fix incorrectly ordered lwkt_reltoken() call in pageout daemon

* The pageout daemon was incorrectly swapping the top two VM objects
  in order to drop the prior object in a specific case where the 'new'
  object being rotated into is NULL.
kernel/u4b: sc is NULL here, so don't dereference it.
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.
Small correction in man5 MLINKS.
varsym.1: Use EXIT STATUS.
mfi(4): Fix a wrong assignment.
ndis(4): Fix a wrong assignment.
kernel/ic: Fix allocation of the output buffer.
libc/regex: Fix setting the maximum of the range.
amd(4): Remove duplicate assignment.
kernel - remove PAGE RACE kprintf

* Used for debugging verification of the race, no longer needed.

Reported-by: tuxillo