An extension to user statuses.

This adds a USERSTATUS_KIND enum to Messenger, which dictates how clients should show user statuses.

A common implementation in a GUI-based client would be indicator lights (green/red/orange), giving a quick indication without having to read the actual status message.

Clients will have to update their calls to m_set_userstatus, and their userstatus callback to take/pass an additional parameter, see the header for details

(this pull request also adds API for querying our own userstatus.)