core/namespace: fix path sorting

The comparison function we use for qsorting paths is overly indifferent.
Consider these 3 paths for sorting:
 /foo
 /bar
 /foo/foo
qsort() may compare:
 "/foo" with "/bar" => 0, indifference
 "/bar" with "/foo/foo" => 0, indifference
and assume transitively that "/foo" and "/foo/foo" are also indifferent.

But this is wrong, we want "/foo" sorted before "/foo/foo".
The comparison function must be transitive.

Use path_compare(), which behaves properly.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1184016
(cherry picked from commit a0827e2b)