Add an option to send datasets with params without preserving encryption

[Idefix2020]

Motivation and Context

Previously it was not possible to send an encrypted dataset with parameters (e.g. -R or -p). This is because the user would usually expect all parameters (including encryption) to be preserved when using the -R or -p flags. For this reason the --raw flag was required on sending with -R or -p to prevent accidentally unencrypting a dataset by sending it to a target without encryption.
Fixes #10507

Description

This PR introduces the --no-preserve-encryption/-U flag to the zfs send command, for if the user explicitly wants to send a dataset without preserving encryption. This prevents anyone from removing encryption on a dataset on accident. Additionally a warning is emmited on sending a previously encrypted dataset without preserving encryption. The idea for an additional flag originally came from #10507.

How Has This Been Tested?

Tested sending encrypted datasets with -R and -p. Sending a dataset now works without --raw if --no-preserve-encryption is specified.

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.

[behlendorf]

Thanks for tackling this much requested bit of functionality. Adding an explicit option to send the dataset unencrypted is a nice way to handle this and enable the use cases discussed in #10507. The PR itself looks reasonable, but it's going to need a test case added to make sure this new option is working properly. Can you add a test case along with the others in tests/zfs-tests/tests/functional/rsend/.

[Idefix2020]

@bbehlendorf Thanks for taking a look at this. I’ll add a test case as soon as possible.

[Idefix2020]

@behlendorf I've added the relevant test case. It looks to be working for me.