Server notices

implements server notices.  Notes:
The umode_s module implementing the +s mode now does something: it manages the categorization and sending of server notices, but does not actually generate them.  It's set up so that other modules can generate them.  This allows greater flexibility in other modules to deliver information to the right places.  This also means that there won't be any notices unless you load other modules for them.  Currently implemented:
- cmd_oper (loaded by default): opers and failed oper attempts
- servnoticeconnect (new): User connect notices
- servnoticequit (new): User quit notices