IMAP SEARCH fixes & FETCH fallback by Sunil Shetye

IMAP SEARCH fixes & FETCH fallback by Sunil Shetye

* The IMAP client now uses "SEARCH UNSEEN" rather than "SEARCH UNSEEN NOT
  DELETED" again on IMAP2, to fix a regression in fetchmail 6.2.5 reported by
  Will Stringer in June 2004. (Sunil Shetye)
* The IMAP client now uses "SEARCH UNSEEN UNDELETED" on IMAP4 and IMAP4r1
  servers (Sunil Shetye).
* Workaround: The IMAP client now falls back to "FETCH n:m FLAGS" if the server
  does not support "SEARCH". (Sunil Shetye)
* The IMAP client now requests message numbers in batches of 1,000 to avoid
  problems if there are more than 1860 unseen messages. (Sunil Shetye)
    Note that this wasn't security relevant because fetchmail would only read up
  to the maximum buffer size and leave the remainder of the string unread, going
  out of synch afterwards.

svn path=/branches/BRANCH_6-3/; revision=5468