do not mark specified references as missing

[takbal]

StaticLint tends to give too many false missing references, and that makes this - otherwise very useful - feature barely usable, as one tends to ignore them, or turn this feature off. Fixing false missing references is always ongoing, but in my experience, this work is progressing very slowly, and new issues keep popping up.

The simple changes in this pull request allow ignoring specific missing reference warnings, via extra comment lines added to any source. These lines should start with the text "#@linter_refs", and then they should contain comma-separated list(s) of the missing references to ignore.

[pfitzseb]

I'm not entirely sure I like this approach, but maybe it's necessary considering how hard it has been to fix the actual issues. Let me think about this a bit more...

[takbal]

Actually, I have done this because I got bored with an issue I have reported 11 months ago, and was not yet touched. And I could report another two since then.

I believe turning off specific warnings from the source is a common feature, see e.g. #pragma in the C++ world.