security/R-cran-openssl: Fix openssl support, IGNORE LibreSSL

The SSL specification wasn't correct. The SSL requirement needs to be
documented even when the base SSL is used. Relocate USES=ssl to be seen
unconditionally and block building on FreeBSD 9 with base OpenSSL using
the standard technique.

LibreSSL is not supported due to use of RSA_get0_* and other unsupported
functions.

Approved by: SSL blanket