Device-independent labels for geom_flashmap slices

[Ian Lepore]

geom_flashmap is inherently based on label names, but it creates devfs
entries for each slice which include the hardware device name.  I've
added support for geom_flashmap to the geom_label class, so that the
labels are also created in /dev/flash without reference to the device
name, similar to how labels work for GPT partitions and filesystems.

I've created a review for the proposed changes at

https://reviews.freebsd.org/D19535

-- Ian