Add an option for entering KDB on recursive panics

Add an option for entering KDB on recursive panics

There are many cases where one would choose avoid entering the debugger
on a normal panic, opting instead to reboot and possibly save a kernel
dump. However, recursive kernel panics are an unusual case that might
warrant attention from a human, so provide a secondary tunable,
debug.debugger_on_recursive_panic, to allow entering the debugger only
when this occurs.

For for simplicity in maintaining existing behaviour, the tunable
defaults to zero.

Reviewed by:	cem, markj
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D27271