Add UNSHARING of filesystems and EXPORTING pools

As a 'stop' action ensure the filesystem is unshared before
it is unmounted, just in case. Additionally, export the pool
so it may be cleanly imported by a different host.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2003