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.