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.
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