kernel - Refactor tty_token, fix SMP performance issues

kernel - Refactor tty_token, fix SMP performance issues

* Remove most uses of tty_token in favor of per-tty tp->t_token.
  This is particularly important for removing bottlenecks related to PTYs,
  which are used all over the place.  tty_token remains in a few places
  managing overall registration and global list manipulation.

* tty structures are now required to be persistent.  Implement a sepearate
  ttyinit() function.  Continue to allow ttyregister() and ttyunregister()
  calls, but these no longer presume destruction of the structure.

* Refactor ttymalloc() to take a **tty pointer and interlock allocations.
  Allocations are intended to be one-time.  ttymalloc() only requires the
  tty_token for initial allocations.

* Remove all critical section use that was combined with tty_token and
  tp->t_token.  Leave only the tokens.  The critical sections were
  hold-overs going all the way back to pre-SMP days.

* syscons now gets its own token, vga_token.  The ISA VGA code and
  the framebuffer code also now use this token instead of tty_token.

* The keyboard subsystem now uses kbd_token instead of tty_token.

* A few remaining serial-like devices (snp, nmdm) also get their own
  tokens, as well as use the now required tp->t_token.

* Remove use of tty_token in the session management code.  This fixes
  a niggling performance path since sessions almost universally go
  hand-in-hand with fork/exec/exit sequences.  Instead we use the
  already-existing per-hash session token.