Add db_rint(), an MI interface to db_enter() copied from kdbrint() in vax code

Add db_rint(), an MI interface to db_enter() copied from kdbrint() in vax code

If ddb.console is set and your serial console driver uses it, db_rint(),
lets you enter ddb(4) by typing the ESC D escape sequence.  This is
useful for drivers like sfuart(4) where the hardware doesn't have a true
BREAK mechanism.

Suggested by miod@, ok kettenis@ miod@