Eugh, this code is getting messy, can somebody verify this regex for me,

Eugh, this code is getting messy, can somebody verify this regex for me,
I'm trying to get it to match on "INBOX" or INBOX but nothing afterwards,
but some IMAP servers are returning an extra char or two on the end, so
a direct match is messing up, so I've tried something a little different
and told it to do a full match, but fail it if the next char after INBOX is
a delimiter (do we have a full list of possibilities?).

Tested on uw, courier, exchange, and cyrus, and the list builds nicely now.
uw still has some odd behaviours when you have a default folder prefix, but
the others seem a lot better... got to do some minor tweaks, and I think this
can go for full testing.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5062 7612ce4b-ef26-0410-bec9-ea0150e637f0