Add -o/--owner to filter files by owner and/or group

[alexmaco]

Closes #307

This implements a single constraint.
Implementation is unix-specific for now. It can easily be extended to cover redox, but I think it needs some testing first: the users crate makes no mention of redox support, and there is also redox_users

Possible questions:

• should this PR also add support for multiple --owner arguments (like with time_constraints)
• help text refinements

Edit: it seems redox uses the unix target_family as of rust-lang/rust#60139, so no special handling is needed.

[sharkdp]

Minor: …either side with a '!' to exclude…

Specifying the full format is great, but a few examples would make things even easier to understand, I think. Maybe:

Examples:
  --owner john
  --owner :students
  --owner '!john:students'

[sharkdp]

Thank you very much for this contribution! The code looks great.

should this PR also add support for multiple --owner arguments (like with time_constraints)

It would be great, but we can also add this at a later stage. Multiple --owner arguments would act in an OR-like fashion, right? Similar to --extension. Maybe we should think about this in a bit more detail. Combining this with negation could lead to surprising results(?).

A few questions / comments:

• Did you do some research into other crates that would work in an OS-independent way? I am not familiar with how file ownership works on Windows, but they certainly have the concept of owners (https://docs.microsoft.com/en-us/windows/win32/secauthz/finding-the-owner-of-a-file-object-in-c--)
• I guess it would be hard to write actual integration tests for this?
• For some reason, the AppVeyor tests did not run for this PR and I can not manually trigger a build. Travis CI tests also didn't run for the last commit. Could you maybe rebase this PR to see if that triggers a build?
• Could you please add an entry to the "unreleased" section in the CHANGELOG.md under "Features"? The format for entries is:

  - Description what has been changed, see #123 (@user)
  

  where #123 links to the bug ticket and/or this PR and user is your username.
• It would be great if you could also update the man page. Otherwise, I can also sync it with the --help text before the next release.

[alexmaco]

Thank you very much for this contribution! The code looks great.
You're welcome!

should this PR also add support for multiple --owner arguments (like with time_constraints)

It would be great, but we can also add this at a later stage. Multiple --owner arguments would act in an OR-like fashion, right? Similar to --extension. Maybe we should think about this in a bit more detail. Combining this with negation could lead to surprising results(?).

Yes, it should work by OR-ing, and negations should work by AND-ing, which i think would make sense when seen in examples, but may not be intuitive to everyone.
I'm waiting for any clear ideas on this topic.

* Did you do some research into other crates that would work in an OS-independent way? I am not familiar with how file ownership works on Windows, but they certainly have the concept of owners (https://docs.microsoft.com/en-us/windows/win32/secauthz/finding-the-owner-of-a-file-object-in-c--)

Unfortunately no, I didn't dig very far, since I can only properly try it out on linux systems. Finding a cross-platform way to do this would be interesting however. I imagine that it could also account for more interesting setups, such as LDAP/ActiveDirectory/whatever-the-equivalent-on-OSX is. But I'm not familiar enough with windows to propose solutions that look idiomatic on the platform.

* I guess it would be hard to write actual integration tests for this?

Integration tests would certainly be interesting. I guess setting up a docker image with known users/groups and IDs would be sufficient. It looks like travis can speak docker.

* For some reason, the AppVeyor tests did not run for this PR and I can not manually trigger a build. Travis CI tests also didn't run for the last commit. Could you maybe rebase this PR to see if that triggers a build?

Sure

* Could you please add an entry to the "unreleased" section in the `CHANGELOG.md` under "Features"? 
...
* It would be great if you could also update the man page. 

Ok, I did not realize more docs existed 😄. I will update them.

[sharkdp]

Thank you very much for the updates!

[sharkdp]

Integration tests would certainly be interesting. I guess setting up a docker image with known users/groups and IDs would be sufficient. It looks like travis can speak docker.

That sounds rather involved. Let's try without an integration test for now.