loader: Use printc instead of print to fit in 24,80

print automatically adds a newline, while printc does not. Use printc in
preference to print for managing the autoboot message. This means we can
use line 24 safely on a 24x80 terminal, restoring some functionality
that was lost in 101afbc6ee2f0.

Note: we still set the default curosor position to 25,1 in screen.lua,
but real VT100s (and successors) will treat any row larger than the
pnumber of rows in a cursor motion command to be the last physical row
(this is so you can move to 9999,9999 and do a cursor location query to
get the size of the screen). Keeping that as is looks better on a
typical VGA console.

Fixes: 101afbc6ee2f0
Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D46771