Scripts Required for ZFS bootloader support

[sugam45]

This PR adds the following scripts to the code base
lz4.h: compression support
zfs_bootenv.h: Define macros for label bootenv nvlist pair keys.
zfs_bootenv_os.h: Define macros for label bootenv nvlist pair keys.
zfssubr.c: checksum, compression and raidz support

[zoulasc]

Why are you doing this piecemeal? I would expect to import all the FreeBSD code needed, and then start to commit on top of it. You also need to mark which revision of FreeBSD you are picking the code from, so in the future we can sync their changes.

[zoulasc]

Ok, now that I've downloaded the diff I can see the changes you've made compared to the FreeBSD original in zfssubr.c. Please fix the PR to first commit all the unmodified FreeBSD code that you'll need. Then issue your changes as a separate pull request so we can comment on them.

1. Do not remove the __FBSDID() but #ifdef notdef it so that we know which version the file came from.
2. Do not make cosmetic/whitespace changes so the code is easier to compare against the original
3. use #ifdef __FreeBSD__/#ifdef __NetBSD__ to make OS dependent changes instead of replacing code (for example in the header files)
4. Does the code even compile? Where is acols defined?