zbm-kcl, zbm-efi-kcl: unify, allow streaming EFI modifications

- Copy necessary functions from zfsbootmenu/lib/zfsbootmenu-kcl.sh to
  avoid a dependency on this file at runtime

- Expand concept of "boot environment" to include "ZBM EFI executable"

- Allow streaming manipulations of EFI executables with "-" indicating
  stdin as a source and stdout as a destination

Author: ahesford