[_WIN32] (posix_mkdir): New.

[_WIN32] (mkdir): Use it.  Last argument is no longer ignored, which
was a bug.  Move declaration to unistd.h, which isn't quite right, but
works for us.

src/lib/w32/unistd.h

@@ -35,4 +35,12 @@
 #define UNISTD_H
 
 #include "getopt.h"
+
+/*
+ * posixfile.c
+ */
+#define mkdir(dir, perm)    posix_mkdir((dir), (perm))
+
+extern int posix_mkdir(const char *dirname, int perm);
+
 #endif /* UNISTD_H */