URL checks

The idea of 'white lists' to prevent known (and 'tolerated') issues, note,
warnings, ... from needlessly reappearing is very powerful and general, and
can go much further than just URL checks.

I suggested several times in the past that we can look at the format Debian
uses in its 'lintian' package checker and its override files -- which are
used across thousands of packages there.  But that went nowhere so I stopped.

This issue needs a champion or two to implement a prototype as well as a
potential R Core / CRAN sponsor to adopt it.  But in all those years no smoke
has come out of any chimneys so ...  ?\_(?)_/? is all we get.

Dirk