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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Turbo Fredriksson <[email protected]>
Signed-off-by: loli10K <[email protected]>
Closes #3238 
Closes #5367 

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