Allow zfs unshare <protocol> -a

Allow zfs unshare <protocol> -a command to share or unshare all datasets
of a given protocol, nfs or smb.

Additionally, enable most of ZFS Test Suite zfs_share/zfs_unshare test cases.
To work around some Illumos-specific functionalities ($SHARE/$UNSHARE) some
function wrappers were added around them.

Finally, fix and issue in smb_is_share_active() that would leave SMB shares
exported when invoking 'zfs unshare -a'

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #3238
Closes #5367