rmdir: work around cygwin 1.5.x and mingw bugs

rmdir: work around cygwin 1.5.x and mingw bugs

* m4/rmdir.m4 (gl_FUNC_RMDIR): Detect the bugs.
* lib/rmdir.c (rmdir): Work around it.
* modules/rmdir (Status, Notice): No longer obsolete.
(Files): Add dos.m4.
(Depends-on): Add unistd.
(configure.ac): Set witnesses.
(License): Relax to LGPLv2+.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Set defaults.
* modules/unistd (Makefile.am): Substitute witnesses.
* lib/unistd.in.h (rmdir): Declare replacement.
* doc/posix-functions/rmdir.texi (rmdir): Document this.
* modules/rmdir-tests: New tests.
* tests/test-rmdir.c: Likewise.

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