[Dovecot] Courier's IMAP custom flags implementation

[Timo Sirainen]

Just thought to mention:

http://dovecot.procontrol.fi/tmp/README.imapkeywords.html

I'm not sure what to think of it. Good thing in it is that it doesn't
limit the number of flags. Bad thing is that I think it's way too slow. 

Compared to Dovecot's implementation where I "steal" maildir flags a..z
and treat them in the normal maildir way. Bad thing is obviously that
it's limited to 26 different flags.

And I really should fix that customflags file soon to be NFS-safe /
read-lockless. It's just a bit problematic how to handle removing unused
flags when you've hit the 26 limit and try to add new ones.