kernel - Add kqueue and fix chflags handling for tmpfs

kernel - Add kqueue and fix chflags handling for tmpfs

* Implement kqueue ops for tmpfs so things like tail -f work properly.

* Redo all the chflags handling and checks to use the helper functions,
  also getting ridding of the horribly broken FREAD/FWRITE tests on
  node->tn_flags (which is the chflags flags, not the open flags).

  This fixes chflags handling for tmpfs.  Previously doing something
  like 'make installkernel ... DESTDIR=<sometmpfs_target>' would fail
  on re-install due to chflags not working properly.

* Fix a lost node unlock in tmpfs_chflags()

Reported-by: Nuno Antunes <nuno.antunes@gmail.com>