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Distinguish between "no partition" and "choose best partition" with a constant.

The values of the d_slice and d_partition fields of a disk_devdesc have a
few values with special meanings in the disk_open() routine. Through various
evolutions of the loader code over time, a d_partition value of -1 has
meant both "use the first ufs partition found in the bsd label" and "don't
open a bsd partition at all, open the raw slice."

This defines a new special value of -2 to mean open the raw slice, and it
gives symbolic names to all the special values used in d_slice and
d_partition, and adjusts all existing uses of those fields to use the new
constants.

The phab review for this timed out without being accepted, but I'm still
citing it below because there is useful commentary there.

Differential Revision:  https://reviews.freebsd.org/D19262

UnifiedSplitRaw