Gnorb nnir engine doesn't take extra criteria

lisp/nngnorb.el: I was misunderstanding the third element of the
		 nnir-engine entry. It's quite likely we could make use
		 of that some other way, in the future, but it's not
		 needed now.
Allow for more "query" types in nnir-run-gnorb

lisp/nngnorb.el: `nnir-run-gnorb' should be able to handle pre-composed
		 lists of links, as well as query strings to run. In
		 some cases we'll want to scan for links elsewhere, and
		 then open up a nnir summary and a BBDB buffer with the
		 results of the same scan.
Let nnir-run-gnorb handle gnus version 5.13

lisp/gnorb-gnus.el: `gnorb-gnus-search-messages' needed a fair amount of
		    reworking to operate correctly under both gnus 5.13
		    and git head. This is why you don't use internal
		    functions!

lisp/nngnorb.el: `nnir-run-gnorb' will get a slightly different format
		 of query string under 5.13.