Fix zmod.h usage in userspace

Do not use zmod.h in userspace.

This has also been filed with the ZFS team. It makes the userspace
libzpool code use the zlib API, instead of the Solaris-only and
non-standard zmod.h. The zlib API is almost identical and is a de
facto standard, so this is a no-brainer.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>