modules/services/printing/cupsd.nix: ensure that /etc/cups exists before the daemon is run

CUPS is really unhappy when /etc/cups doesn't exist, and without that
directory it's going to mess up file permissions of /nix/store, the web
front-end won't work, and probably all kinds of other stuff is broken
that I didn't notice yet. :-(

svn path=/nixos/trunk/; revision=29452

modules/services/printing/cupsd.nix

@@ -133,6 +133,7 @@ in
 
         preStart =
           ''
+            mkdir -m 0755 -p /etc/cups
             mkdir -m 0755 -p ${logDir}
             mkdir -m 0700 -p /var/cache/cups
             mkdir -m 0700 -p /var/spool/cups