SSH: Introduce keep-alive mechanism.

Neither TCP nor the SSH protocol offer a built-in way to reliably
notice connection loss, so we implement our own.

Task-number: QTCREATORBUG-3783

src/plugins/coreplugin/ssh/sshoutgoingpacket.cpp

@@ -130,6 +130,16 @@ void SshOutgoingPacket::generateRequestFailurePacket()
     init(SSH_MSG_REQUEST_FAILURE).finalize();
 }
 
+void SshOutgoingPacket::generateIgnorePacket()
+{
+    init(SSH_MSG_IGNORE).finalize();
+}
+
+void SshOutgoingPacket::generateInvalidMessagePacket()
+{
+    init(SSH_MSG_INVALID).finalize();
+}
+
 void SshOutgoingPacket::generateSessionPacket(quint32 channelId,
     quint32 windowSize, quint32 maxPacketSize)
 {