careadlinkat: stop exporting careadlinkatcwd

careadlinkat: stop exporting careadlinkatcwd

Only Emacs used it directly, and Emacs no longer needs it.
* NEWS: Document this simplification.
* lib/areadlink.c (careadlinkatcwd): Move here from careadlinkat.c,
and make it static.  Include <stdlib.h>, for abort, and unistd.h,
for readlink.
* lib/careadlinkat.c (careadlinkatcwd): Move to areadlink.c.
Don't include stdlib.h; no longer needed.
* lib/careadlinkat.h (careadlinkatcwd): Remove decl.
* lib/relocwrapper.c: Adjust comment to match new dependencies.
* modules/areadlink (Depends-on): Add readlink.
(Maintainer): Add self.
* modules/careadlinkat (Depends-on): Remove readlink.