mac: kdb/ddb framework hooks
Add three simple hooks to the debugger allowing for a loaded MAC policy
to intervene if desired:
- Before invoking the kdb backend
- Before ddb command registration
- Before ddb command execution
We extend struct db_command with a private pointer and two flag bits
reserved for policy use.
Reviewed by: markj
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35370