The traditional behavior while opening freebsd MBR slice has been to try
to open BSD partition 'a' first, if that fails, open MBR slice.
The implementation of this behavior has been in disk_open(), but it does
conflict with behavior described in common/disk.h and this update does
implement the feature in devopen() instead.
After this patch, we can update disk_open() to behave as described in
common/disk.h.