Extensible Link Support

Continue the work that was started with the creation of dbLink.{h,c} towards making link types extensible.

This involves more refactoring of the code that handles links, and probably looking hard at the link address parsing code to see if it can be changed to offer each link to the different types in turn until one of them accepts it.

We will have to continue parsing the existing fixed link addresses for backwards compatibility reasons, but future link types should use a standard syntax, preferably JSON since we already have a parser for it in base-3.15.