MPSAFE TTY - Refactor kprintf()'s spinlock, shutdown, move cons_spin

MPSAFE TTY - Refactor kprintf()'s spinlock, shutdown, move cons_spin

* Adjust panic assertions to reduce reentrant panics.

* Redo the shutdown code to properly interlock panic_cpu_gd

* Move cons_lock into subr_prf.c and use a normal spinlock instead of
  a deprecated (w/interrupt disablement) spinlock.

  Adjust kvcprintf() when used with kputchar() to acquire cons_spin
  only if the current cpu is not the currently panicing cpu.

  Adjust kvcprintf() when used with kputchar() to acquire a hard
  critical section.