Fix handling of NMIs from unknown sources (BMC, hypervisor)

Fix handling of NMIs from unknown sources (BMC, hypervisor)

Release kernels have no KDB backends enabled, so they discard an NMI
if it is not due to a hardware failure.  This includes NMIs from
IPMI BMCs and hypervisors.

Furthermore, the interaction of panic_on_nmi, kdb_on_nmi, and
debugger_on_panic is confusing.

Respond to all NMIs according to panic_on_nmi and debugger_on_panic.
Remove kdb_on_nmi.  Expand the meaning of panic_on_nmi by making
it a bitfield.  There are currently two bits: one for NMIs due to
hardware failure, and one for all others.  Leave room for more.

If panic_on_nmi and debugger_on_panic are both true, don't actually panic,
but directly enter the debugger, to allow someone to leave the debugger
and [hopefully] resume normal execution.

Reviewed by:	kib
MFC after:	2 weeks
Relnotes:	yes: machdep.kdb_on_nmi is gone; machdep.panic_on_nmi changed
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D24558