Dedup IOC enum values in libzfs_input_check

Reuse enum value ZFS_IOC_BASE for ('Z' << 8).
This is helpful on FreeBSD where ZFS_IOC_BASE has a different value and
('Z' << 8) is wrong.

Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9188