golangci-lint: enable more linters and address linting issues

[thaJeztah]

• follow-up to https://github.com/opencontainers/selinux/pull/189

(more coming after this, but I have ETOOMANYBRANCHES already)

[rhatdan]

I am ok with almost all of these, although I am not crazy about the err:= shadow issue. I find it a lot easier to read

if err := foobar(); err != nil }

Then err := foobar() if err != nil { }

Bottom line, I think err should be handled differently then other shadowed vars. But I am willing to live with the changes.

LGTM

[thaJeztah]

I am not crazy about the err:= shadow issue.

Yes, I'm slightly on the fence on that one as well;

• in most cases, it's mostly "noise"
• ^^ that said, I'm not against doing a "once in a while" round on changing them
• ^^^ but ... that said; I've also encountered situations where an err was masking an err that was checked in a defer(). Admitted, those situations are rare (and usually better to have a distinct name for "special" errors).

I don't think the linter itself has a config for this, but perhaps a string match exclusion would work (see https://golangci-lint.run/usage/false-positives/#exclude-issue-by-text)

[rhatdan]

Tests are failing for go-fumpt?

[thaJeztah]

Tests are failing for go-fumpt?

I probably need to rebase see https://github.com/opencontainers/selinux/pull/189#issuecomment-1277485676

[thaJeztah]

Rebased, and tweaked the settings to ignore "err" for shadowing 👍 PTAL

[thaJeztah]

Oh! Had one more change staged in another branch; just pushed that one as well.

[rhatdan]

Nice work @thaJeztah