security/dsniff: unbreak for FreeBSD 12+

This software needs old SSL interfaces that pre-date OpenSSL 1.1
and provided by LibreSSL still.

Unbreak build for systems using openssl 1.1 following technique
of security/acme-client: compile with temporary copy of LibreSSL
and statically link binaries sshmitm and webmitm with LibreSSL's
libraries. Do not install LibreSSL to the system
if SSL_DEFAULT is not libressl.