kdb: Modify securelevel policy

Currently, sysctls which enable KDB in some way are flagged with
CTLFLAG_SECURE, meaning that you can't modify them if securelevel > 0.
This is so that KDB cannot be used to lower a running system's
securelevel, see commit 3d7618d8bf0b7. However, the newer mac_ddb(4)
restricts DDB operations which could be abused to lower securelevel
while retaining some ability to gather useful debugging information.

To enable the use of KDB (specifically, DDB) on systems with a raised
securelevel, change the KDB sysctl policy: rather than relying on
CTLFLAG_SECURE, add a check of the current securelevel to kdb_trap().
If the securelevel is raised, only pass control to the backend if MAC
specifically grants access; otherwise simply check to see if mac_ddb
vetoes the request, as before.

Add a new secure sysctl, debug.kdb.enter_securelevel, to override this
behaviour. That is, the sysctl lets one enter a KDB backend even with a
raised securelevel, so long as it is set before the securelevel is
raised.

Reviewed by: mhorne, stevek
MFC after: 1 month
Sponsored by: Juniper Networks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37122