MFC r273178: Update vt(4) for UEFI defaults and special keys

  vt(4) is the default console for UEFI boot [1], and the bitmapped
  kern.vt.spclkeys sysctl has been replaced with individual kern.vt.kbd_*
  enable sysctls.

PR:		193710
MFC r273219: Do nothing in vt_upgrade if there is no vt driver

  Previously, if no drivers attached at boot we would panic with
  "vtbuf_fill_locked begin.tp_row 0 must be < screen height 0".

PR:		192248