even if not asked for with the "return (special-use)" extended list parameter.
macos x mail does not request the special-use flags, but will use them when present.
for issue #66, thanks x8x for providing the imap protocol transcript that
showed how it is done!
a few places still looked at the name "Sent". but since we have special-use
flags, we should always look at those. this also changes the config so admins
can specify different names for the special-use mailboxes to create for new
accounts, e.g. in a different language. the old config option is still
understood, just deprecated.
