flag stuff

[Simon Josefsson <jas@pdc.kth.se>]

Perhaps I ought to explain how the flag stuff works now, since this is
work-in-progress:

The summary hooks save a copy of the Group Info when you enter a
folder, and when you quit the folder it compare the new & old Group
Info and propagate all changes to the server. When entering a group,
the Group Info is also updated from the server.

The real work (setting the flags) is done by the new backend function
`nnimap-request-set-mark' and in the future, hopefully, Gnus will call
this function directly with the marks that should be set instead of
letting theese cludgy hooks do the work.

I'm working on the Gnus patches, they turned out to be a little bit
trickier than I thought<tm>...

/s