MFC r352194: lualoader: Revert to ASCII menu frame for serial console

The box drawing characters we use aren't necessarily safe with a serial
console; for instance, in the report by npn@, these were causing his xterm
to send back a sequence that lua picked up as input and halted the boot.
This is less than ideal.

Fallback to ASCII frames for console with 'comconsole' in it. This is a
partial revert r338108 by imp@ -- instead of removing the menu entirely and
disabling color/cursor sequences, just reverting the default frame to ASCII
is enough to not break in this setup.