NetworkStream now rejects non-stream sockets

- Constructor throws IOException when socket type is not Stream, matching .NET.
- Remove unreachable datagram branches from Read and Write.
- Remove redundant socket type check from SslStream constructor.
- Fix exception documentation (drop nonblocking clause, stray characters, wrong conditions).
- Add NetworkStream constructor tests.

Author: josesimoes

nanoFramework.System.Net/Sockets/NetworkStream.cs

@@ -71,9 +71,6 @@ public NetworkStream(Socket socket, bool ownsSocket)
                 throw new IOException(errCode.ToString(), e);
             }
 
-            // NetworkStream is only valid for stream-oriented sockets (same as .NET).
-            // Datagram sockets don't have stream semantics: e.g. a zero-length datagram
-            // would make Read return 0 without the peer having shut down the connection.
             if (socket.SocketType != SocketType.Stream)
             {
                 throw new IOException();
@@ -341,4 +338,3 @@ public override int Read(SpanByte buffer)
         }
     }
 }
-