build - refactor rescue and initrd handling

build - refactor rescue and initrd handling

* buildworld now builds the rescue directory tree and initrd
  image in /usr/obj.

* installworld will install the rescue and initrd stuff if it
  does not exist on the destination.  If rescue or initrd stuff
  is already installed, it is not overwritten.

* Add 'installrescue' and 'installinitrd' targets.  These will
  copy the rescue and initrd image built by buildworld.

* The normal 'rescue' and 'initrd' targets are unchanged and
  should build (from scratch) and install the rescue and initrd
  stuff.

* NOTE: crunchgen is currently hacked to add system paths when
  WORLDBUILD is defined, in order to access native binaries
  for 'uniq' and a few others, instead of building them in
  cross-tools.

  In particular, we have to use the native 'vnconfig' binary
  as building one in crosstools might not be compatible with
  the currently running system.  buildworld now uses 'vn' to
  create the initrd image.

  Possibly needs some work.