boot(9), reboot(9): Migrate MD callers of boot(9) to reboot(9)

boot(9), reboot(9): Migrate MD callers of boot(9) to reboot(9)

I have found that some ports call boot(9) from machine-dependent code to
reboot system.  These should be changed to either:

- Sending signal to init(8) to trigger it to shutdown system cleanly, like
  acpi(4) does, in cases where found problems don't prevent system from working
  immediately, or

- Just doing panic(9) if the situation is severely broken.

For now, just rewrite boot() to reboot().  Actual fixes follow.

Discussed with & OK from kettenis@