fix defn of mkbox.  use mdir by default.

Reference: /n/atom/patch/applied/mkbox
Date: Sat Mar  1 00:43:49 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /acme/mail/mkbox	Sat Mar  1 00:43:34 2014
+++ /acme/mail/mkbox	Sat Mar  1 00:43:34 2014
@@ -2,8 +2,9 @@
 
 for(i){
 	if(! test -e $i){
-		if(mkdir $i){
-			chmod 0773 $i
+		if(cp /dev/null $i){
+			chmod 600 $i
+			chmod +al $i
 		}
 	}
 	if not echo $i already exists