Add zilstat script to report zil kstats in a user friendly manner

[ixhamza]

Motivation and Context

Script for zil kstats

Description

Python script to process both global and per dataset zil stats and report them in a user-friendly manner similar to arcstat and dbufstat.

How Has This Been Tested?

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Performance enhancement (non-breaking change which improves efficiency)
• [ ] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• [ ] Documentation (a change to man pages or other documentation)

Checklist:

• [ ] My code follows the OpenZFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [x] I have read the contributing document.
• [ ] I have added tests to cover my changes.
• [ ] I have run the ZFS Test Suite with this change applied.
• [x] All commit messages are properly formatted and contain Signed-off-by.

[ixhamza]

cc: @amotin, @freqlabs.

[ghost]

It would be helpful to print the numeric field headers with "/s" when an interval is given to indicate that the values printed are per second, not per interval. For example, "zcc/s" (but not "ds/s").

[ixhamza]

Thanks, @richardelling for your review, I have updated the code according to your suggestions.