If the IMAP server supports the UID EXPUNGE command, the nnimap backend
currently always expunges deleted articles.

If the IMAP server does not support this command, articles are either
not expunged at all (nnimap-expunge is nil), or ALL articles that are
currently flagged as deleted will be expunged when any one article is
deleted in Gnus.

This patch introduces three new settings for nnimap-expunge:

 * If nnimap-expunge is 'never, deleted articles are marked with the
   IMAP \\Delete flag but not automatically expunged.

 * If nnimap-expunge is 'immediately, deleted articles are immediately
   expunged (this requires the server to support the UID EXPUNGE
   command).

 * If nnimap-expunge is 'on-exit, deleted articles are flagged, and all
   flagged articles are expunged when the group is closed.

Note that none of these behaviors is available with the current settings
(nil / non-nil).

The 'on-exit setting is mostly useful as a safe-guard: as long as the
group is not closed, article deletions are reversal (either by using a
different IMAP client, or by exiting Gnus without updating the summary
buffer).

The 'never setting is not (yet) very useful because Gnus does not
distinguish between deleted articles and regular articles in the summary
buffer. However, this will be addressed by a separate patch.

Changes in v2:

  * Made first line of docstring a complete sentence.

Changes in v3:

  * Correctly handle multiple active nnimap backends.

Thanks for considering,
-Nikolaus

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