allow nvr to be in the dos active partition so the fs can be booted
without a floppy drive.

the changes work like this
1. make sure _sdinit() is always called before we try to look up a drive.
2. make sure that _sdinit actually sees the drives available.  perhaps the
check to make sure drives are not added twice should be moved to
_sdinit from *pnpsd.  this would make drive deconfiguration and
reconfiguration easier.
3. noisy debugging changes.  sorry.