kernel - Add EVFILT_USER

* Bring in EVFILT_USER support from FreeBSD.  Document the hacked use of
  EV_CLEAR.

* Implemented a bit differently.  Currently not implemented as another
  filter callback.  Instead we explicitly test for user_filtops, which
  isn't as messy (at least until some other kqueue feature needs the 'touch'
  interface).

Taken-From: FreeBSD