MFC r248068 (by sbruno):

MFC r248068 (by sbruno):
Add legacy support to geom raid to create a /dev/arX device for support
of upgrading older machines using ataraid(4) to newer releases.

This optional parameter is controlled via kern.geom.raid.legacy_aliases
and will create a /dev/ar0 device that will point at /dev/raid/r0 for
example.

Tested on Dell SC 1425 DDF-1 format software raid controllers installing from
stable/7 and upgrading to stable/9 without having to adjust /etc/fstab

git-svn-id: svn://svn.freebsd.org/base/stable/9@251916 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f