[DDMD] Add ddmd manually ported source and build system support

@WalterBright @andralex @AndrejMitrovic @MartinNowak @9rnsr

Finally, the time has come to start moving forward with this.

This pull adds a new make target 'ddmd' which converts the source over and builds it with the just-build C++ dmd.

The most important file is `settings.json` which contains the type lists and mappings from C++ source to D source.

To build, you need [magicport](https://github.com/yebblies/magicport2) built.

Run `make ddmd MAGICPORT=..\path\to\magicport2\magicport2.exe` (`MAGICPORT` defaults to `..\..\magicport2\magicport2.exe`)

Some of the source remains stubbed out (response, libmach, etc) and some files are missing headers and other cleanup, but this is a purely additive change and will hopefully encourage some more devs experiment with ddmd and test their pull requests against it.

Also contains #3409 until that's merged.