stand: Introduce new dv_parsedev routine

Allow device classes to define a parsing routine. Most device classes
already have these routines, but there's much duplication in their
use. Define an interface for a common routine to parse an individual
device. By convetion, files have the form "[device:]/path/to/file"
where device is optional (filled in to be the value of currdev)
and it starts with the dv_name field of the device, with the rest
of the name up to the device (typically a unit number, but disks
add partition inforation, and other devices may do artibtrary
otehr things).

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37337