FreeBSD doesn't have a `share` command. It does have showmount.
Split the separate platform impls out of is_shared_impl.
Dispatch to the correct platform impl function from is_shared.
Eliminate the use of is_shared_impl from tests. is_shared works.
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes#10037
b7dbbf6aa7