Start to reduce the number of #ifdef EFI_ZFS_BOOT

There's a number of EFI_ZFS_BOOT #ifdefs that aren't needed, or can be
eliminated with some trivial #defines. Remove the EFI_ZFS_BOOT ifdefs
that aren't needed. Replace libzfs.h include which is not safe to
include without EFI_ZFS_BOOT with efizfs.h which is and now
conditionally included libzfs.h. Define efizfs_set_preferred away
and define efi_zfs_probe to NULL when ZFS is compiled out.