remove: new module, for mingw and Solaris 9 bugs

remove: new module, for mingw and Solaris 9 bugs

Mingw obeys C89, but not POSIX, by not handling directories.
Solaris remove("file/") mistakenly succeeded.

* modules/remove: New file.
* lib/remove.c: Likewise.
* m4/remove.m4 (gl_FUNC_REMOVE): Likewise.
* m4/stdio_h.m4 (gl_STDIO_H_DEFAULTS): Add witnesses.
* modules/stdio (Makefile.am): Use them.
* lib/stdio.in.h (remove): Declare replacement.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/remove.texi (remove): Likewise.
* modules/remove-tests: New test.
* tests/test-remove.c: Likewise.

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