Generate unavailable presence

[jonkri]

RFC 6121 states:

Before ending its presence session with a server, the user's client SHOULD gracefully become unavailable by sending "unavailable presence", i.e., a presence stanza that possesses no 'to' attribute and that possesses a 'type' attribute whose value is "unavailable". [...] Optionally, the unavailable presence stanza MAY contain one or more elements specifying the reason why the user is no longer available. [...] However, the unavailable presence stanza MUST NOT contain the <priority/> element or the <show/> element, since these elements apply only to available resources.