MFC r291377: vidfont: with vt(4) omit size from vidcontrol -f

When using syscons, vidfont extracts the font size from the filename
passes it to vidcontrol -f. In vt(4) mode the size argument is not
required, and some of the fonts in /usr/share/vt/fonts do not have the
size in the filename, which caused vidfont to fail. Thus, just omit the
size argument in vt(4) mode.