[Mono.Debugger.Soft] Make sure receiving thread always closes

When we want to close the connection, setting the close flag is not enough, because a thread can be already stuck in a blocking receive, without getting to check the flag. We fix this by additionaly breaking the connection, by using socket.Shutdown.

Fixes mono#7377

mcs/class/Mono.Debugger.Soft/Mono.Debugger.Soft/Connection.cs

@@ -1318,6 +1318,8 @@ class PacketWriter {
 		protected abstract int TransportSend (byte[] buf, int buf_offset, int len);
 		protected abstract void TransportSetTimeouts (int send_timeout, int receive_timeout);
 		protected abstract void TransportClose ();
+		// Shutdown breaks all communication, resuming blocking waits
+		protected abstract void TransportShutdown ();
 
 		internal VersionInfo Version;
 
@@ -1434,6 +1436,7 @@ class PacketWriter {
 
 		internal void Close () {
 			closed = true;
+			TransportShutdown ();
 		}
 
 		internal bool IsClosed {
@@ -2865,6 +2868,11 @@ protected override void TransportClose ()
 		{
 			socket.Close ();
 		}
+
+		protected override void TransportShutdown ()
+		{
+			socket.Shutdown (SocketShutdown.Both);
+		}
 	}
 
 	/* This is the interface exposed by the debugger towards the debugger agent */