(1) Use namei() and devfs to discover devices rather than a hard-coded
MAKEDEV implementation. Once rootfs is in place, this will allow
Vinum to be used for the root file system partition.
(2) Pass FREAD to device opens so that GEOM will return sector size
rather than an error on attempts to read label data.
(3) Avoid clobbering return values from close_drive() and masking this
failure, resulting in a later divide by zero due to not having
updated the Vinum-cached sector size.
(4) Ignore failures from DIOCWLABEL as that appears not to be required
in the GEOM environment.
We've done testing in simple Vinum environments, but those with more
complex environments might want to give this a spin in DP2 and make
sure everything is up to speed.
Fixes in collaboration with: iedowse
Reviewed by: grog
722fc75788