Consolidate GetUserName code

Once .NET Core 1.1 becomes available, we can likely remove the entirety
of CorePsExtensions.cs.

test/csharp/test_CorePsPlatform.cs

@@ -34,7 +34,7 @@ namespace PSTests
                 // The process should return an exit code of 0 on success
                 Assert.Equal(0, process.ExitCode);
                 // It should be the same as what our platform code returns
-                Assert.Equal(username, Platform.NonWindowsGetUserName());
+                Assert.Equal(username, Platform.Unix.UserName());
             }
         }