I'd like to do the following:

1) change `d_flags' field in bdevsw to `d_type', and add a similar
field in cdevsw.

2) Add a set of constants, currently D_{TAPE,DISK,TTY}.  D_TAPE should
take the place of B_TAPE, which is currently used in d_flags, and
D_DISK and D_TTY should be specified for appropriate devices.

3) Make isdisk() and istty() machine-independent, and add istape().
It occurs to me that perhaps these should just be deleted instead,
though, and their usages changed to directly access the switch table.

Comments?  If I don't hear objections, I'm going to do this RSN.