take pkg_install out of the base system and pre-install it using the

take pkg_install out of the base system and pre-install it using the
pkgsrc binary packages.

rationale:
  . pkg_install (which is the pkg_* tools) is entangled with pkgsrc,
    not with minix, so tracking it from pkgsrc (easier than with
    base system) makes more sense
  . simplifies upstreaming minix specific changes for pkg_* tools
  . reduce pkgsrc-in-basesystem maintenance burden