Disable colors if NO_COLOR env var is present and nonempty

[egmontkob]

Proposed changes

Disable colors when environment variable NO_COLOR is present and nonempty, according to the "standard" described in http://no-color.org/

Checklist

• I have referenced the issue(s) resolved by this PR (if any)
• I have signed-off my contribution with git commit --amend -s
• Lint and unit tests pass locally with my changes (make indent && make check)
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation (if appropriate)

---

This was already submitted a long time ago, at MidnightCommander/mc-old#133. Then it was closed without mc devs expressing their opinions.

Since then, a couple of things have changed:

• The standard fixed the handling of the env var's empty value.
• They clarified that app-specific options should be able to override it.
• They sort of clarified a few things on their homepage (it's not super-duper obvious, but I have the strong impression that they're fine with attributes like reverse video, but ANSI colors (even grayscale) are not okay.
• mc's black and white mode was fixed recently, and it happens to match this previous requirement.
• mc's development has moved here to github.
• The number of supporting projects listed on the no-color homepage has grown from 3 to 257.

NO_COLOR is still a somewhat controversial initiative, several people have expressed that it should have been done differently, but it's definitely gaining traction. With about 4 lines of source code, why not join them?

@RauliL I've prepared the change in your name as the original author. I had to modify two things: the handling of the empty value, and moving the code elsewhere so that --color can override it. Neither were your fault: the standard changed since then.

It's signed-off-by me. Not sure how it should be done ideally, but I don't think I should add someone else's name there. Please let me know if you have any comment or preference; maybe the cleanest would be if you could take back full ownership and re-submit the PR fully under your name?

[zyv]

Given the evolution of the NO_COLOR project, I wouldn't object on principle.

[egmontkob]

Friendly pings

@mc-worker Review please

@RauliL Any feedback? I'll submit on your behalf in about a week if I don't hear back from you.

[RauliL]

@egmontkob I'm OK with this.

[egmontkob]

I realized I did not add documentation, so added now.

We don't have a dedicated "env vars" section and I didn't want to introduce one, so I've added it to the description of the -b option, which is a bit unusual choice. Let me know if you guys would prefer a different solution.

[zyv]

Let me know if you guys would prefer a different solution.

I'm still fine with that.

[mc-worker]

We don't have a dedicated "env vars" section and I didn't want to introduce one

#3733