stand/zfs: Limit flags further for ZFS

Constrain CFLAGS for ZFS: don't add anything globally. Add the includes
to only the files that need them. Add -DHAS_ZSTD_ZFS to zfs.c (which
includes zfsimpl.c which includes zfssubr.c both of which need this
defined). Also add it to efi/boot1/Makefile since zfs_module.c also
includes zfsimple.c.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35887