linux emu - peripheral changes

linux emu - peripheral changes

* These are peripheral changes to prepare for the import of the updated
  linuxulator:

* hooks (eventhandler) for process creation and process exit.

* splitting up of sys_kevent into sys_kevent + kern_kevent, including
  generic copyiin and copyout functions.
  Obtained-from: FreeBSD

* adding a p_textnch to store the namecache handle for the .text file.

* minor changes to sysv_sem and sysv_shm to support linux-specific
  magic.

* bring in casuword (compare and set userland word)
  Obtained-from: FreeBSD

* add a hook for userret from syscall2
  Inspired-by: NetBSD