I've commited changes to the ddb.4 manual page that document the machine
specific commands that I found by source code inspection. It's a first
draft; I specifically invite people to review it. It'll be in tomorrow's
sup. Ideally, this closes PR#1562 (now in "feedback" state, waiting on
y'all).

In addition, Jonathan Stone has suggested that a "halt" command be added to
the generic DDB command set, with a rough interpretation of "return control
to whatever PROM monitor exists on this machine".

It's not clear to me that enough machines have such a thing for this to be
useful - only five of our platforms even have machine-specific commands,
though there are some cute ones in there that might also be candidates for
inclusion in generic DDB, subject to "code bloat" objections.

However, some rationalization of the machine specific commands is probably
in order (sparc says "prom" when everyone else uses "halt"). Now that
they're all documented side-by-side, comparisons can be made.

	your thoughts & suggestions,

	Erik <fair@clock.org>