mkdir, mkdirat: fix cygwin 1.5.x bug

mkdir, mkdirat: fix cygwin 1.5.x bug

mkdir("dir/./",mode) mistakenly succeeded.

* lib/mkdir.c (rpl_mkdir) [FUNC_MKDIR_DOT_BUG]: Work around bug.
* m4/mkdir-slash.m4 (gl_FUNC_MKDIR_TRAILING_SLASH): Move...
* m4/mkdir.m4 (gl_FUNC_MKDIR): ...here, and add check for cygwin
bug.
(gl_PREREQ_MKDIR): Delete unused macro.
* modules/mkdir (Files): Track file rename.
(configure.ac): Update macro name.
* modules/openat (Depends-on): Add mkdir.
* doc/posix-functions/mkdir.texi (mkdir): Document the bug.

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