Update zfs-install

Multiple changes:

1. Switch from gzip-9 to LZ4
2. Use /tmp/zpool.cache
3. Remove unnecessary keywording
4. Use emerge @module-rebuild
5. Replace reboot with export
6. Keyword ZFS and GRUB:2 on stable
7. Apply use flag on grub:2.
8. Set bootfs

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>