Implement server notices (usermode +s)

This implements the server notice mechanism. It adds umode +s which acts as a listmode and keeps track of what users are subscribed to what events. It supports using multiple mode parameters using commas and can list what events the user is subscribed to. Oper permissions are split into different permissions for each event with a "servernotice-all" wildcard permission.
We could also do that.  In that case, this entire check is redundant.