Fixes Issue 1219

At cketti's suggestion, change the methodology for counting
server-side unread messages on an IMAP server from STATUS (UNSEEN) to
SEARCH UNSEEN NOT DELETED.  STATUS (UNSEEN) does not ignore DELETED
but unexpunged messages and leads to incorrect results.

http://www.mail-archive.com/c-client@u.washington.edu/msg00929.html
http://www.mail-archive.com/c-client@u.washington.edu/msg00931.html

This variant only checks for unread messages in the last 300 messages
in the folder, so will be wrong if there are unread messages earlier
in the folder.  However, it'll be more often correct than the old
version for the marjority of users.

Also, change the opening of folders for IMAP IDLE purposes to
read-only using EXAMINE instead of SELECT.