loader: bring back old text-only video console

Bring back vidconsole.c as textvidc.c from 2a0e2c88db20. This console
does no graphics stuff at all, supports no fancy logos, has known bugs
in the terminal emulation, etc. However, it is small. It will be a
build-time option to select between the two. The BIOS loader is running
out of space when too many options are selected, so this allows people
to select the smaller one to spend the space elsewhere. This is only the
verbatim copy of the old vidconsole.c. It's not yet connected to the
build.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43911