stand: More sensible defaults when ConOut is missing

When ConOut is missing, we used to default to serial. Except we did it
in the worst way possible by just setting the howto bits and not
updating the console setting, which lead to weird behavior where we'd
get some things on the video port, others on serial.

Instead, set console to "efi,comconsole" for this case. Also set
RB_MULTIPLE always (so we get dual consoles from the kernel) and or in
RB_SERIAL when we can't find GOPs that suggest the precense of a video
console. This will put output in the most places and have a sensible
default for 'primary' console.

Sponsored by: Netflix
Reviewed by: emaste, manu
Differential Revision: https://reviews.freebsd.org/D36299