DEVFS - Refactor vinum to operate with devfs
* vinum now accepts devtab labels and device paths (such as serial numbers)
in drive specifications. Note that devtab labels will be convreted to
device serial numbers in the on-disk configuration.
* vinum now creates and destroys its own devices via DEVFS.
* Fix some buf/bio issues related to recent buf/bio work. bio_done is
required for ad-hoc buffers such as the kind vinum uses, and also
required for any synchronous IO.
* Remove adhoc device associations. Instead store the device returned
by make_dev() in appropriate internal structures.