Add the new cdldr CD bootstrap loader. This patch includes the following:

Add the new cdldr CD bootstrap loader.  This patch includes the following:

- Fix btxldr to preserve a NULL bootinfo pointer when it copies the kernel
  arguments.
- Add the cdldr bootstrap program.  This program is tacked onto the
  beginning of the standard 3rd stage boot loader (/boot/loader) to form
  the CD boot loader (/boot/cdboot).  When a CD is booted, the cdboot file
  is copied into memory instead and executed.  The cdldr stub emulates the
  environment normally provided by boot2 and then starts the loader.  This
  booting method does not emulate a floppy drive, but boots directly off of
  the CD.  This should fix the problems some BIOS's have with emulating a
  2.88 MB floppy image.
- Add support to the loader to recognize that it has been booted by cdldr
  instead of boot2 and use a simpler method of extracting the BIOS boot
  device.