Group storage and vmail user options at mailserver.storage

Create a nicer option structure that deals with the mail storage and its
owner, uid, group and gid. Also includes the directory layout as a
property of how mails are stored..

mail-server/rsnapshot.nix

@@ -61,7 +61,7 @@ in
         retain	hourly	${toString cfg.backup.retain.hourly}
         retain	daily	${toString cfg.backup.retain.daily}
         retain	weekly	${toString cfg.backup.retain.weekly}
-        backup	${cfg.mailDirectory}/	localhost/
+        backup	${cfg.storage.path}/	localhost/
       '';
     };
   };