Refine and update blacklist support in sshd

Adjust notification points slightly to catch all auth failures,
rather than just the ones caused by bad usernames.

Modify notification point for bad usernames to send new type of
BLACKLIST_BAD_USER. (Support in libblacklist will be forthcoming soon.)
Add guards to allow library headers to expose the enum of action values.

Reviewed by: des
Approved by: des
Sponsored by: The FreeBSD Foundation