Add aliasing concept to geom.

Add an alias name list to geoms. Use them in geom_dev to create
aliases. Previously, geom_dev would create an device node for the name
of the geom. Now, additional nodes are created pointing back to the
primary node with make_dev_alias_p. Aliases must be in place on the
geom before any tasting occurs.

Differential Revision: https://reviews.freebsd.org/D11873