linkat: new module

linkat: new module

* modules/linkat: New file.
* lib/at-func2.c (at_func2): Likewise.
* lib/linkat.c (linkat): Likewise.
* m4/linkat.m4 (gl_FUNC_LINKAT): Likewise.
* lib/openat-priv.h (at_func2): Add declaration.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witnesses.
* modules/unistd (Makefile.am): Substitute them.
* lib/unistd.in.h (linkat): Declare it.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/linkat.texi (linkat): Likewise.
* doc/posix-functions/link.texi (link): Tweak wording.
* tests/test-link.c (main): Move guts...
* tests/test-link.h (test_link): ...into new file.
* modules/linkat-tests: New test.
* tests/test-linkat.c: Likewise.
* modules/link-tests (Files): Ship new file.
(Depends-on): Add stdbool.

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