- Add kern.fwd.mk to generate forward headers for missing machine/*.h

- Add kern.fwd.mk to generate forward headers for missing machine/*.h
  It avoids unnecessary regeneration of forward headers by default, thus
  quickkernel is speedy again :)
- Nuke no longer needed generate_forwarding_headers() in config(8)
- Add forward headers generation to the dependency list of "beforedepend"
  in kmod.mk.  This unbreaks module building which does not happen along
  with kernel building.

The original patch was submitted by y0netan1@, changes made to the original
patch are OKed by him too.

Reviewed-by: dillon@ corecode@