New GUIDs for Lidgren.Network.{Linux,MacOS}.csproj

These two projects had the same GUID as Lidgren.Network.Windows.csproj
did, which caused trouble when trying to include them in the same
solution.

This gives the Linux and MacOS projects new GUIDs and updates the
solutions that include them to use the new GUID.
Merge pull request #2196 from andrewschleifer/lidgren-guid

New GUIDs for Lidgren.Network.{Linux,MacOS}.csproj