symlink: new module, for Solaris 9 bug
symlink("a","link/") mistakenly succeeds.
* modules/symlink: New file.
* m4/symlink.m4 (gl_FUNC_SYMLINK): Likewise.
* lib/symlink.c: Likewise.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add defaults.
* modules/unistd (Makefile.am): Substitute them.
* lib/unistd.in.h (symlink): Declare replacement.
* MODULES.html.sh (File system functions): Mention it.
* doc/posix-functions/symlink.texi (symlink): Likewise.
* modules/symlink-tests: New test.
* tests/test-symlink.c: Likewise.
Signed-off-by: Eric Blake <ebb9@byu.net>