[PATCH] Move clause_sides_match_join() into pathnode.h

We had two almost-identical copies of the utility function
clause_sides_match_join() -- one in joinpath.c, and one in
analyzejoins.c. Both copies were marked "inline," so we might as well
just move the (inline) function definition into a common header file.
I chose pathnode.h, because it is already #included by both of the .c
files.

This change doesn't help very much, on its own (since the
almost-duplicate functions are both "inline"), but it allows us to use
the same utility function from other source files, without making a
third copy.

James