Use an unique directory name instead of hardcoded /tmp/.diskless.

A malicious user could create a file named /tmp/.diskless and cause
the script to misbehave.

PR:		conf/141258
Reported by:	Jon Passki
MFC after:	1 week

etc/rc.d/tmp

@@ -51,8 +51,8 @@ case "${tmpmfs}" in
 [Nn][Oo])
 	;;
 *)
-	if /bin/mkdir -p /tmp/.diskless 2> /dev/null; then
-		rmdir /tmp/.diskless
+	if _tmpdir=$(mktemp -d -q /tmp/.diskless.XXXXXX); then
+		rmdir ${_tmpdir}
 	else
 		if [ -h /tmp ]; then
 			echo "*** /tmp is a symlink to a non-writable area!"