mapping from numbers to names is messy for backwards compatibility.
E.g., for driver "sd", unit "0":
slice 0: omit the slice number for compatibility; names are sd0[a-h].
slice 1: omit the partition letter 'c' because the whole disk device
shouldn't have anything to do with partitions; sd0 is the
only name.
slices 2-31: subtract 1 from slice number to compensate for the
compatibility slice 0; names are sd0s[1-30][a-h].
For it to be useful, you must stick your disklabel on the partition which
starts where the MBR says FreeBSD lives. If you don't do that, you might
get a bad day.
Oh, that probably also means that putting swap there is a bad idea...
