Add a very limited DDB dumpon(8)-alike to MI dumper code

This allows ddb(4) commands to construct a static dumperinfo during
panic/debug and invoke doadump(false) using the provided dumper
configuration (always inserted first in the list).

The intended usecase is a ddb(4)-time netdump(4) command.

Reviewed by: markj (earlier version)
Differential Revision: https://reviews.freebsd.org/D21448