loader: Move drawer.lua over to gfx table.

Drawer.lua is the only bit of lua code in the base that uses any of the
functons moved from the loader table to the gfx table. Move the main
code to using the gfx dispatch. Add compat code for running on old
loaders that creates the newer-style gfx table with the term_* functions
we call in it populated. This will even work on the super old versions
of the loader that don't have them (we'll still skip using them).

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D43908