pruneNonExisting: handle eselect-opengl symlinks

pruneNonExisting: handle eselect-opengl symlinks

Only count symlinks as preserved if they still point to a hardink
in the same directory, in order to handle cases where a tool such
as eselect-opengl has updated the symlink to point to a hardlink
in a different directory (see bug #406837). The unused hardlink is
automatically found by _find_unused_preserved_libs, since the soname
symlink no longer points to it. After the hardlink is removed by
_remove_preserved_libs, it calls pruneNonExisting which eliminates
the irrelevant symlink from the registry here.