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/systemd.nix

@@ -46,7 +46,7 @@ in
       preStart =
         let
           directories = lib.strings.escapeShellArgs (
-            [ cfg.mailDirectory ] ++ lib.optional (cfg.indexDir != null) cfg.indexDir
+            [ cfg.storage.path ] ++ lib.optional (cfg.indexDir != null) cfg.indexDir
           );
         in
         ''
@@ -55,7 +55,7 @@ in
           # Prevent world-readable paths, even temporarily.
           umask 007
           mkdir -p ${directories}
-          chgrp "${cfg.vmailGroupName}" ${directories}
+          chgrp "${cfg.storage.group}" ${directories}
           chmod 02770 ${directories}
         '';
     };