disable proxy domain options in tweak settings if mod_proxy is not available

Since we know that mod_proxy support is required for proxy subdomains to work (we document this in the interface), and it's easy to check and see if the support is compiled into apache (httpd -M | grep proxy), I would like to have the options for proxy subdomains greyed out if they are not available because mod_proxy isn't installed.

This is really easy to overlook, can waste quite a bit of time, and is move towards our new general helpfullness of greying out things that can't be used.