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 of: r273544