ddb: add _FLAGS command variants

Provide _FLAGS variants of the various command definition macros, in
anticipation of adding a new flag. This can also be used for some
existing commands which require special flag values.

Reviewed by: markj
MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35581