readlinkat: split into its own module

readlinkat: split into its own module

* modules/symlinkat: Split readlinkat...
* modules/readlinkat: ...into separate module.
* m4/symlinkat.m4 (gl_FUNC_SYMLINKAT): Move readlinkat check...
* m4/readlinkat.m4 (gl_FUNC_READLINAT): ...to new file.
* lib/symlinkat.c (readlinkat): Move...
* lib/readlinkat.c: ...into new file.
* modules/symlinkat-tests: Split readlinkat test...
* modules/readlinkat-tests: ...into separate module.
* tests/test-symlinkat.c: Split...
* tests/test-readlinkat.c: ...into new file.
* NEWS: Document the split.
* doc/posix-functions/readlinkat.texi (readlinkat): Likewise.
* lib/unistd.in.h (readlinkat): Likewise.
Suggested by Bruno Haible.

Signed-off-by: Eric Blake <eblake@redhat.com>