Keeping the spl/zfs module split, libzpool is the zfs module for
userspace. Headers and functions specific to it belong there.
Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes#17861
