MFC automatic vt(4) selection for UEFI boot

MFC automatic vt(4) selection for UEFI boot

r268158: Prefer vt(4) for UEFI boot

  The UEFI framebuffer driver vt_efifb requires vt(4), so add a
  mechanism for the startup routine to set the preferred console.
  This change is ugly because console init happens very early in the
  boot, making a cleaner interface difficult.  This change is intended
  only to facilitate the sc(4) / vt(4) transition, and can be reverted
  once vt(4) is the default.

r268160: Fix typos in VTY constant names from r268158

r268982: Don't pass null kmdp to preload_search_info

  On Xen PVH guests kmdp == NULL.

Sponsored by:	The FreeBSD Foundation