consider a out-of-range partition letter at the end of the special
device node to be a fatal error; just return NULL and let the caller
fall back to FFS.
This fixes the "mount -u /kern/rootdev /" done by the script installer.
XXX this is still gross, and breaks things like
"mount /my/strange/dev/path/b /mnt". Perhaps it should stat the node
and use the minor number as an index instead?
be856ef968