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. It also implements the following snomasks:
- connect
- quit
- remoteconnect
- remotequit
Your subscribe list will also include remote users who probably don't want to receive one of these messages per server.  Do we want to filter them?  (Probably when we're adding to the subscribe list, below, rather than here.)