mac: kdb/ddb framework hooks

[mhorne]

Summary

Add three simple hooks to the debugger allowing for a loaded MAC policy
to intervene if desired:

1. Before invoking the kdb backend
2. Before ddb command registration
3. Before ddb command execution

We extend struct db_command with a private pointer and two flag bits
reserved for policy use.

[markj]

sys/ddb/db_command.c

481

This line appears to be too long.

sys/security/mac/mac_debugger.c

1

mac_kdb.c might be a better name for this file. "debugger" sounds too generic to me.