net/SocketDescriptor: add SetTcpUserTimeout()

src/net/SocketDescriptor.cxx

 /*
- * Copyright (C) 2012-2017 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -316,6 +316,13 @@ SocketDescriptor::SetTcpDeferAccept(const int &seconds) noexcept
 }
 
 bool
+SocketDescriptor::SetTcpUserTimeout(const unsigned &milliseconds) noexcept
+{
+	return SetOption(IPPROTO_TCP, TCP_USER_TIMEOUT,
+			 &milliseconds, sizeof(milliseconds));
+}
+
+bool
 SocketDescriptor::SetV6Only(bool value) noexcept
 {
 	return SetBoolOption(IPPROTO_IPV6, IPV6_V6ONLY, value);

src/net/SocketDescriptor.hxx

 /*
- * Copyright (C) 2012-2017 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -188,6 +188,12 @@ public:
 	bool SetCork(bool value=true) noexcept;
 
 	bool SetTcpDeferAccept(const int &seconds) noexcept;
+
+	/**
+	 * Setter for TCP_USER_TIMEOUT.
+	 */
+	bool SetTcpUserTimeout(const unsigned &milliseconds) noexcept;
+
 	bool SetV6Only(bool value) noexcept;
 
 	/**