rss: configurable announce method

rss: configurable announce method

Since the RSS update announcements do not expect any form of reply,
they could be considered the typical IRC message that should use
NOTICE rather than PRIVMSG.

However, for backwards compatibility and since NOTICEs are not always
appreciated (and since their handling from clients is not always
optimal, either), we still allow rss announces to use the traditional
method, and that is in fact selected as default. Explicit rss show
request always go with PRIVMSG.