DEFAULT_VERSIONS+=ssl=openssl to /etc/make.conf as a workaround.

Squid 3.5 requires pre-1.1 versions of OpenSSL, and that can be only
satisfied by the base system on 11.x versions of FreeBSD; doing the
DEFAULT_VERSIONS+=ssl=openssl would not work.  Unfortunately, making
proper conversion from CRYPTO_LOCK_SSL_* to new multi-threading API
does not look feasible.  Fortunately, LibreSSL could be used as a
drop-in replacement.

That said, change the advice to use DEFAULT_VERSIONS+=ssl=libressl
instead, and add a couple of minor patches to unbreak the build.