nnir.el and process marks

[Kai.Grossjohann@CS.Uni-Dortmund.DE]

I hear that it is very important for nnir to only search the
process-marked groups because searching all groups takes too long a
time on an IMAP server.

Several people have implemented patches to nnir for specific mail
backends in the past, but I need mechanisms which in principle can
work for all mail backends.  So I did not include Simon's patch for
searching the process-marked groups only; instead I have come up with
the following code.

Sadly, I don't use IMAP yet, and thus I can't test this feature.

Could somebody please have a look at this to see what needs to be done
to make it work?

Attention: The following code has never been tested at all!

Thanks,
kai
-- 
I like _b_o_t_h kinds of music.

nnir.el