Write support for the cd(4) driver.

Write support for the cd(4) driver.

This allows writing to DVD-RAM, PD and similar drives that probe as CD
devices.  Note that these are randomly writeable devices, not
sequential-only devices like CD-R drives, which are supported by cdrecord.

Add a new flag value for dsopen(), DSO_COMPATLABEL.  The cd(4) driver now
uses this flag instead of the DSO_NOLABELS flag.  The DSO_NOLABELS always
used a "fake" disklabel for the entire disk, provided by the caller.

With the DSO_COMPATLABEL flag, dsopen() will first search the media for a
label, and if it finds a label, it will use that label.  Otherwise it will
use the fake disklabel provided by the caller.  This provides backwards
compatibility, since we will still have labels for ISO9660 media.

It also provides new functionality, since you can now have a regular BSD
disklabel on read-only media, or on writeable media (e.g. DVD-RAM).

Bruce and I both think that we should eventually (in a few years) get
away from using disklabels for ISO9660 media, and just use the whole disk
device (/dev/cd0).  At that point disklabel handling in the cd(4) driver
could follow the "normal" model, as used in the da(4) driver.

Also, clean up the path in a couple of places in cdregister().  (Thanks to
Nick Hibma for catching that bug.)

Reviewed by:	bde