Track upstream project rename in contrib/blocklistd

Upstream is now https://github.com/zoulasc/blocklist/. Rename the
contrib directory and update Makefiles to match, in advance of the next
vendor branch update.

Sponsored by: The FreeBSD Foundation

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Add basic blacklist build support

Reviewed by: rpaulo
Approved by: rpaulo
Relnotes: YES
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5913