MFC r281581: vidcontrol: make size argument optional again for syscons

r273544 (MFC in r273921) changed the -f option allow no arguments in vt
mode (used to reset the font back to the default), but broke the
optionality of the size argument for syscons. Drop the required argument
from syscons' optstring for -f so the optional argument handler works
the same way for both syscons and vt.

Sponsored by: The FreeBSD Foundation