Introduce D label prefix for scratch raids

Rationale:

SW-RAIDs are assembled by serials,
Serials are kept in a database,
thus they need a uniqe label.

Since this applies also for level-0 scratch raids, and a clash with
decent SW-RAIDs (M-prefix) is unwanted, they get the D-prefix.

mxmount

@@ -169,6 +169,9 @@ sub parse_data {
                if (!$data[3]) {
                    $D->{noexport} = 1;
                }
+           } elsif($D->{label} =~ /^D/) {             # D: scratch software raid (mdadm)
+               $D->{mountpoint} = 'D/' . $D->{label};
+               $D->{label} = lc($D->{label});
            } elsif($D->{label} =~ /^M/) {             # M: decent software raid (mdadm)
                $D->{mountpoint} = 'M/' . $D->{label};
                $D->{label} = lc($D->{label});