areadlinkat: new module

areadlinkat: new module

* lib/at-func.c (FUNC_FAIL): New define.
(AT_FUNC_NAME, VALIDATE_FLAG): Use it rather than raw -1.
* modules/areadlinkat: New module.
* lib/linkat.c (areadlinkat): Move...
* lib/areadlinkat.c (areadlinkat): ...to new file.
* lib/areadlink.h (areadlinkat): Declare it.
* modules/linkat (Depends-on): Add areadlinkat.
* MODULES.html.sh (File system functions): Mention it.
* modules/areadlinkat-tests: New test.
* tests/test-areadlinkat.c: New file.

Signed-off-by: Eric Blake <ebb9@byu.net>