Rename nodes/relation.h => nodes/pathnodes.h ?

In the pluggable-storage discussion, there was some talk of renaming
nodes/relation.h to avoid confusion with the new access/relation.h
header. I think this is a fine idea, not only because of that conflict
but because "relation.h" has never made a lot of sense as the file's
name.

After a bit of thought, I propose "pathnodes.h" as the new name.
That fits in with the other major headers in that directory
(primnodes.h, parsenodes.h, plannodes.h, execnodes.h), and it seems
like a reasonable summary of what's in it. Admittedly, Path nodes
as such are barely a third of the file's bulk; but I don't see any
equally pithy way to describe the rest of it, unless something like
planner_data.h, which is pretty unmelodious.

(There was some mention of trying to split relation.h into multiple
files, but I fail to see any advantage in that.)

Barring objections, I'm happy to go make this happen.

regards, tom lane