Incompatible with Curl NSS backend

Curl can use NSS as the crypto backend. Unfortunately xbmc hard codes a default cipher list string that is only compatible with OpenSSL and GnuTLS. As there is no common default, we should simply avoid touching the cipher list except when we want something explicit.

Red Hat uses Curl with NSS, so this affects quite a few users.

Branch is against Gotham, not master.