X-DOVECOT capabilities

[Timo Sirainen]

Dovecot has a few hidden features currenly, which I know some webmails have been hardcoded to use. Maybe it would be nice to have these available in a bit more globally usable way and advertise some kind of X-DOVECOT capability? I don't really like the idea of having a ton of separate X-DOVECOT-* capabilities. Maybe instead just have X-DOVECOT-1 for the current functionality and when more is added X-DOVECOT-2 etc? Then describe in Dovecot wiki page what they actually contain. Thoughts?

The current list of features are at least:

 - STATUS (X-SIZE): Sum of all RFC822.SIZEs in a mailbox. (More efficient than FETCHing them.)
 - STATUS (X-GUID): Show mailbox's GUID.

 - SEARCH X-SAVEDBEFORE, X-SAVEDON, X-SAVEDSINCE: Search with message's saved-timestamp.
 - FETCH X-SAVEDATE: Fetch message's saved-timestamp.

 - SEARCH & FETCH X-GUID: Search/fetch message GUID

 - SEARCH X-MAILBOX, FETCH X-MAILBOX, FETCH X-REAL-UID: With virtual mailboxes search/fetch the mail's physical mailbox name and message UID.

 - X-CANCEL <tag>: Stop a (long-running) command, which once in a while goes back to the main loop. For example SEARCH or FETCH.