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>