kernel - Use per-cpu token for deadlwps list

* There is a deadlwps reaper thread per cpu, use a per-cpu token instead
  of a global token to control it.
kernel - Fix hammer recovery crash (due to recent syncer work)

* Unconditionally create a syncer thread for each mount.  This way we can
  create the thread prior to calling VFS_MOUNT.

* hammer(1) needs to acquire vnodes and potentially issue vn_rdwr()'s during
  mount for recovery purposes.  This syncer thread is expected to already
  exist.  (and it does now).

* Remove the default syncer thread.

* rewrite speedup_syncer().
kernel - Fix bug last commit

* Don't add the syncer vnode to the syncer list if the mount point
  has no syncer thread (optimization for nullfs).
kernel - Fix bug last commit (2)

* Oops. and don't try to get the syncer thread's context if there is no
  syncer thread for a mount point.  I'm sure I'll get this right.
kernel - more vfs syncer stuff

* Make sure we stop the thread when a mount attempt fails.