Fix Issue 9728 - Ddoc anchors non-unique across overloads 

https://issues.dlang.org/show_bug.cgi?id=9728

This keeps the first anchor the same for hyperlink compatibility, subsequent anchors get `.N` appended.

The Jump-to JS code for Phobos will need updating, I'll do that next.
Why the need for global variables? They're nothing but trouble.