Check for sane $_SERVER variables in hook_requirements

A client was hosting using a company called Pantheon that seems to use Varnish, Nginx, and FCGI. This combination produced an empty $_SERVER['SERVER_NAME'] variable which is guaranteed to cause OAuth signatures to fail (used in OAuth.php library).

This patch adds a few more sanity checks to the hook_requirements to alert people about their config. Debugging OAuth signature mis-matches is no fun.