ddb(4): Add 'show badstacks' command to show witness badstacks

Add a DDB command that mirrors sysctl debug.witness.badstacks.

Reapply r323935 after fixing trivial deficiency. I forgot to compile with
WITNESS enabled. Thanks emaste@ for fixing the build while I was asleep.

Reported by: rstone
Reviewed by: rstone (previous version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12468