stand: Print a warning of the loader is too old

If the loader is < 3.0, print a warning that it's too old and needs to
be upgraded.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D45889

(cherry picked from commit a2fd7ae87991044ca283974ced8f6ed635d1cb0c)