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
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25865

(cherry picked from commit e426c74375ef7cb3afda3c8e3010a7ea2dbd69d0)