reboot(9), panic(9): Call panic(9) for unrecoverable MD H/W errors (NMIs)

reboot(9), panic(9): Call panic(9) for unrecoverable MD H/W errors (NMIs)

Some architectures have ability to detect hardware sanity and notify system
(NMI, firmware callback, etc.).  Handle these hardware severe errors, same
as software errors, with panic(9).

According to miod@, SGI IP27 NMI is triggered by pushing some "hidden" button,
which "usual" users/admins don't know.  Pushing such a button is "RB_USERREQ"
(human-triggered) in that the button is pushed by a human, but not "RB_USERREQ"
in that no user intervention in system (== no command input) is done.  miod@
agreed that changing these from RB_USERREQ to !RB_USERREQ (== panic(9)) is
not a big problem.

OK miod@ kettenis@