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