apache-httpd: harden default SSL cipher list

A couple of tweaks on the SSL cipher list.

Disabled RC4 which is now considered broken.
https://community.qualys.com/blogs/securitylabs/2013/03/19/rc4-in-tls-is-broken-now-what

Enabled Forward Secrecy for modern browsers.
https://en.wikipedia.org/wiki/Forward_secrecy

Without the change, NixOS servers are capped at Grade B on
https://www.ssllabs.com/ssltest/index.html