flags in the disklabel. These flags were discarded in 2006.
Remove documentation for and instances of 'ty'. Add instances of
'dt' that were missing. Use 'floppy' as the value of 'dt' for all
floppies and 'rdroot' for all boot images/miniroots.
Add 'rdroot' to dktypenames and an associated #define DTYPE_RDROOT,
since 'dt' values are parsed by matching entries in dktypenames[].
Slightly tweaked part of larger cleanup diff previously ok deraadt@.

disklabel the "HP C3010 / Cranel 2.0GB Fast SCSI2 H10" crammed inside your
palm device, but that's the price of progress.  hopefully i didn't whack
anything important.
ok deraadt