sanoid #912: sanoid --prune-snapshots performance boost by removing u…

[sdettmer]

…nneeded iszfsbusy()

for #912

Option 3) dev/sde/remove-force-prune - If we think the whole functionality is not needed, best would to remove it and keep things more clean. Maybe this could be done in a later version, when experiences shows, that no one ever used --no-force-prune or --double-check-prune. Doing this later probably is too much, but I work in an industry with 20 years warranty periods so I'm used to think this way :)

[phreaker0]

@jimsalterjrs made the necessary changes, this is now ready to be merged

[lckarssen]

Just adding a reminder that the Wiki page needs to be updated as well once this is merged.