vt(4): Refuse to load a font if hw.vga.textmode is selected

Before, the font was loaded and the window size recalculated, giving an
unusable terminal, even if the actual font didn't change.

Reported by:	beeessdee@ruggedinbox.com
MFC of:		r273330

sys/dev/vt/vt_core.c

@@ -2200,6 +2200,9 @@ vtterm_ioctl(struct terminal *tm, u_long cmd, caddr_t data,
 	case PIO_VFONT: {
 		struct vt_font *vf;
 
+		if (vd->vd_flags & VDF_TEXTMODE)
+			return (ENOTSUP);
+
 		error = vtfont_load((void *)data, &vf);
 		if (error != 0)
 			return (error);