Improve libzfs api for zfs diff and add JSON support

[anodos325]

This commit adds json output support for zfs-diff and expands the libzfs API to include an easier-to-use function for generating a diff programmatically. JSON zfs-diff streams output with one JSON object per line.

Motivation and Context

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.
• 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.
• All commit messages are properly formatted and contain Signed-off-by.

[anodos325]

root@testMW87SLWSMH[~/zfs]# zfs diff -j dozer/foo@now1
{"change_type":"modified","inode":2,"gen":6215,"file_type":"directory","path":"/mnt/dozer/foo/d1","changes":{"mode":{"from":16877,"to":16832}}}
{"change_type":"modified","inode":34,"gen":6213,"file_type":"directory","path":"/mnt/dozer/foo/"}
{"change_type":"modified","inode":128,"gen":6216,"file_type":"regular","path":"/mnt/dozer/foo/d1/f1"}
{"change_type":"renamed","inode":129,"gen":6218,"file_type":"regular","path":"/mnt/dozer/foo/f2","changes":{"path":"/mnt/dozer/foo/f3"}}
{"change_type":"added","inode":130,"gen":6254,"file_type":"regular","path":"/mnt/dozer/foo/d1/f5"}

^^^ sample output.

I included what additional information I could glean with existing diff ioctl into the JSON output. This means catching:

• txg / inode generation + inode number (useful for uniquely identifying files)
• changes in mode (potentially useful for catching when permissions on file is changing -- though not authoritative if ACLs are enabled)

This was mainly an exercise in getting zfs diff output available in a useful way for truenas_pylibzfs, but I thought having JSON output implemented would provide better test coverage of the libzfs_iter_diffs (new function) and give some new useful knobs for regular consumers of zfs-diff.