lang/referenceassemblies-pcl: add new port

You can build .NET apps across a wide variety of platforms, and the
Portable Class Library (PCL) helps you share your code and libraries across
.NET platforms. Specifically, the PCL provides a set of common reference
assemblies that enable .NET libraries and binaries to be used on any .NET
based runtime - from phones, to clients, to servers and clouds

Notes:

• The official reference assemblies come from a Microsoft download page.
• Additional reference assemblies come from Debian (via Mono).
• Bump PORTREVISION to indicate we are not using the pure PCL from Microsoft
• Provide a plain-text version of the EULA that is dialog(1) friendly.
• The official RTF based EULA is distributed alongside the LICENSE file