MFC r318242: 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
Relnotes: yes
Sponsored by: The FreeBSD Foundation