Add alias support to gpart.

When we're creating new providers for each of the partitions, add
aliases to the geom before we create the provider so when geom_dev
tastes the provider, the aliases are in place so the proper /dev
entries are created. So foo5p6 gets created as an alias for bar5p6
when foo is an alias for bar in the geom we're partitioning with
g_part. This also copies aliases from the container geom (eg disk) to
the label geom (the disk with GPT partitioning) so that aliases nest
properly.

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