I'm migrating to Gnus, and I'm in need for non-ASCII folder names.

   I've found them not supported (they just didn't appear in *Gnus
Browse Server*) so I made a copy of nnimap.el and discovered that though
some support for UTF-7 was in place, it was not working as expected
(particularly, double-encoded UTF-7 strings happened).  Investigating
which `utf7-encode' or `utf7-decode' were missing and which superfluous
went difficult, so I made a redesing: everything in nnimap deals with
multibyte, except for two functions most close to the IMAP stream.

   I almost succeeded: the only `utf7-encode' found it's place in
`nnimap-send-command' and `utf7-decode' — in `nnimap-parse-response',
*Gnus Browse Server* displays all of the folders with correct message
count (with a hack from http://www.emacswiki.org/emacs/GnusNiftyTricks).
However I still can't enter non-ASCII folders, and if I subscribe to
them, neither can I enter nor can I see message count.  This has
something to do with UTF-8 unibyte strings which spring at me when I try
to enter or when I subscribe.  I assume this has something to do with
Gnus<+>nnimap interaction, though I don't know what.

   Here goes the patch to be applied explicitly to `nnimap.el'
(generated with `diff -u' against gnus in emacs tree of 2010-11-22),
which is mostly clean, no debugging `message' calls left, except for the
function `nnimap-possibly-change-group' still trying unsuccessfully to
deal with UTF-8 unibyte input (`decode-coding-string' works, but
debugged `nnimap-send-command' reports that it receives unibyte
nonetheless).