loader: Add loader.exit

Add loader.exit(status). While one can get alomst this behavior with
loader.perform("quit"), quit doesn't allow a value to be returned to the
firmware. The interpretation of 'status' is firmware specific. This can
be used when autobooting doesn't work in scripts, for example, to allow
the firmware to try something else...

Sponsored by: Netflix

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44094

loader: Move gfx functions to gfx.lua.8

Now that the fb_* and term_* functions are available in the gfx table,
move the documentation to gfx.lua.8. Add information about backwards
compatibility.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D43907

loader: Document the lua loader table.

Document all the public functions from the "loader" table.

Sponsored by: Netflix
Reviewed by: pauamma_gundo.com, tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D43701