xrefslave: true, but only for selected groups

[Ivan Shmakov]

Oh, that message should actually read:

	I have a strange wish: could there be a way to set ``xrefslave:
	true'' on a per-group basis?

	The idea is to ``mirror'' some of the gmane.* groups; N most
	recent articles just now, but getting even the oldest ones
	eventually.  Quite obviously, it would be better to number
	articles according to their injection time stamp (since NNTP
	clients are usually relying on this), and the most natural way
	to do it is just to re-use Gmane's own article numbers, as
	specified by Xref:.

PS.  I know, it would be a dirty hack.