loader: Fix boot menu on BIOS

[imp]

Summary

Only the gfx-enabled boot loader supports unicode. Otherwise, we have to
use the old cons25 / ibmpc upper code page drawing characters. Check to
see if we have the gfx.term_drawbox function. If we do, we support the
unicode drawing characters. If we don't, then we have an older loader
that doesn't support it *OR* we have the reduced function, text-only
boot loader. Either way, we need to use the old graphics characters.

Sponsored by: Netflix

[dhw]

Tested successfully on a laptop: first, running stable/14-n269448-4e8444d5750a, then running main-n273387-bb8b3b174118.

[imp]

Update to move these knowledge into core.hasUnicode() in case
other issues like this crop up, or in case we export this knowledge
more directly.

[imp]

better comment
also remove comment from where we create old bindings:
with this new setup they aren't relevant.

[emaste]

stand/lua/core.lua

548

[imp]

fix ed's typo

[zarychtam_plan-b.pwste.edu.pl]

Thank you for coming up with the solution. I can confirm that it fixes the issue for sc(4). Please don't hesitate to push it, preferably shortly.
Edit:
Wow.... already committed. Impressive timings!