Add method for writing server notices.

This allows us to send a server notice to a user without worrying
about whether they are registered or not.

If a user receives a server notice and they are not registered
then the nickname field will contain an asterisk instead of their
nick name.

We previously sent 'Auth' instead of an asterisk but this was a
non-standard feature which there was no consensus on how it must
be implemented.

This change modifies the way we implement it to be the same as
Charybdis. This is more sensible because:

1. It is used for a similar job by other parts of the protocol.
2. It is not a valid nickname character.