> FreeBSD succeeds with mkdir /tmp/foo/.  So does NetBSD -current from
> the second week in august.

because the mkdir command trims the trailing / before passing the
pathname to the syscall:

% ktrace mkdir /tmp/zzz/
% kdump | grep NAMI
 23330 ktrace   NAMI  "/usr/local/bin/mkdir"
 23330 ktrace   NAMI  "/usr/bin/mkdir"
 23330 ktrace   NAMI  "/bin/mkdir"
 23330 mkdir    NAMI  "/tmp/zzz"
 23330 mkdir    NAMI  "/tmp/zzz"

which is because of the following code in mkdir.c:

		/* Remove trailing slashes, per POSIX. */
		slash = strrchr(*argv, '\0');
		while (--slash > *argv && *slash == '/')
			*slash = '\0';
	
				- Bill