restrict XMLRPC to configurable groups and users

Ignore-this: 1370cee84d44bb35ae7c0c988ed8f1ff

This patch adds a config option to define users or groups that may access
the XMLRPC API. ACLs are still checked on top of this intial access control.

darcs-hash:20090516114351-7ad00-155fb6c74c29f5f84e79544a534369eec0403ff0.gz