We should replace avisynth.org in avisynth.h header file

[pinterf]

It looks like the link in avisynth.h file (http://www.avisynth.org/) is no longer valid and points to an unrelated page.

It should be replaced with some valid link. Or we should just delete it?

[qyot27]

It was valid at one time, of course, but hosting shifted some time ago to avisynth.nl (which itself redirects to the main page of the Wiki).

Even 2.6 moved the url in internal.h to *.nl back in 2015:
451a232

So I would say we probably should do that across the codebase, with the possible exception of some of the references in the documentation that should probably either be removed entirely, or in some cases, it may be appropriate to keep the reference to *.org for posterity but add a (defunct) after it.

[pinterf]

Duplicate of #319