Update to Dovecot 1.1.15.

v1.1.15 2009-05-17  Timo Sirainen <tss@iki.fi>

+ IMAP: When multiple commands are pipelined, try harder to combine their
  mailbox syncing together. For example with Maildir pipelining STORE 1:*
  +FLAGS \Deleted and EXPUNGE commands the files won't be unnecessarily
  rename()d before being unlink()ed.
- IMAP: SEARCH command was slower than necessary in a large mailbox.
- deliver: When forwarding messages, if -f parameter was given, it should have
  been used as the Return-Path.
- Maildir saving: Fixed race condition bugs in uidlist handling, causing files
  to be given new UIDs sometimes.
- mbox: Don't crash when expunging all messages and file doesn't end with
  [CR]LF.
- expire-tool: Use mail_uid and mail_gid settings if userdb doesn't return
  uid/gid.
- Berkeley DB dict: Transaction rollbacking was implemented wrong.
- zlib plugin: Some email / FETCH command combinations could have disconnected
  the client or sent compressed garbage.

(ghen)