vt(4): Add PIO_VFONT_DEFAULT ioctl to restore the default builtin font

To restore the default font using vidcontrol(1), use the "-f" flag
without an argument:
    vidcontrol -f < /dev/ttyv0

PR:		193910
Differential Revision:	https://reviews.freebsd.org/D971
Submitted by:	Marcin Cieslak <saper@saper.info>
Reviewed by:	ray@, emaste@
Approved by:	ray@
MFC after:	1 week