unlink: new module, for Solaris 9 bug unlink("file/") mistakenly succeeded. This patch favors, but does not enforce, GNU semantics that unlink("link-to-dir/") flat-out fails rather than attempting to unlink "dir". * modules/unlink: New file. * lib/unlink.c: Likewise. * m4/unlink.m4 (gl_FUNC_UNLINK): Likewise. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witnesses. * modules/unistd (Makefile.am): Use them. * lib/unistd.in.h (stat): Declare replacement. * MODULES.html.sh (systems lacking POSIX:2008): Mention module. * doc/posix-functions/unlink.texi (unlink): Likewise. * modules/unlink-tests: New test. * tests/test-unlink.c: Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>