[Dovecot] v1.2.3 release candidate

http://dovecot.org/tmp/dovecot-1.2.3.rc.tar.gz

I thought about doing v1.2.3 release immediately, but I think I'll wait about 10-24h first. So until then, testing this RC is appreciated and hopefully we'll have a good release this time. :)

+ Allow namespace prefix to be opened as mailbox, if a mailbox
  already exists in the root dir.
- Maildir: dovecot-uidlist was being recreated every time a mailbox
  was accessed, even if nothing changed.
- listescape plugin was somewhat broken
- Compiling fixes for non-Linux/BSDs