sshd: allow UseBlocklist alias for UseBlacklist

blacklistd has been renamed to blocklistd upstream, and a future
import into FreeBSD will follow that change. Support the new name
as an alias in config files.

Reviewed by: bz, delphij
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25865