areadlink, areadlinkat: rewrite in terms of careadlinkat

areadlink, areadlinkat: rewrite in terms of careadlinkat

* lib/areadlink.c, lib/areadlinkat.c: Include careadlinkat.h
instead of errno.h, limits.h, stdint.h, stdlib.h, string.h, unistd.h.
(SSIZE_MAX, INITIAL_BUF_SIZE): Remove.
(malloc, realloc): Remove #undefs.
(areadlink, areadlinkat): Rewrite in terms of careadlinkat.
* modules/areadlink (Depends-on): Add careadlinkat.  Remove
readlink, ssize_t, stdint, unistd.
* modules/areadlinkat (Depends-on): Add careadlinkat.  Remove
areadlink, stdint.

careadlinkat: new module
* lib/allocator.h, lib/careadlinkat.h, lib/careadlinkat.c:
* modules/careadlinkat: New files, written by me with
a review and feedback from Ben Pfaff in
<http://lists.gnu.org/archive/html/bug-gnulib/2011-04/msg00008.html>.