imap: LIST shows "INBOX" in all caps

While selecting a mailbox is done case-insensitively, "INBOX" is
special for the LIST command, according to RFC 3501 6.3.8:

> The special name INBOX is included in the output from LIST, if
> INBOX is supported by this server for this user and if the
> uppercase string "INBOX" matches the interpreted reference and
> mailbox name arguments with wildcards as described above.  The
> criteria for omitting INBOX is whether SELECT INBOX will
> return failure; it is not relevant whether the user's real
> INBOX resides on this or some other server.

Thus, the existing news.public-inbox.org convention of naming
newsgroups starting with "inbox." needs to be special-cased to
not confuse clients.

While we're at it, do not create ".0" for dummy newsgroups if
they're selected, either.

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