mkinitrd(8): Stop copying regular binaries to the ramdisk

mkinitrd(8): Stop copying regular binaries to the ramdisk

* initrd binaries are specially tailored versions and don't necessarily
  work in the same way as regular ones

* In fact, regular binaries from newer DragonFly systems are expected to
  use dynamic libraries and won't work at all in the fully static environment
  of the ramdisk