core/socket: allow MPTCP protocol

Multipath TCP (MPTCP), standardized in RFC8684 [1], is a TCP extension
that enables a TCP connection to use different paths. It allows a device
to make use of multiple interfaces at once to send and receive TCP
packets over a single MPTCP connection. MPTCP can aggregate the
bandwidth of multiple interfaces or prefer the one with the lowest
latency, it also allows a fail-over if one path is down, and the traffic
is seamlessly re-injected on other paths.

To benefit from MPTCP, both the client and the server have to support
it. Multipath TCP is a backward-compatible TCP extension that is enabled
by default on recent Linux distributions (Debian, Ubuntu, Redhat, ...).
Multipath TCP is included in the Linux kernel since version 5.6 [2]. To
use it on Linux, an application must explicitly enable it when creating
the socket:

  int sd = socket(AF_INET(6), SOCK_STREAM, IPPROTO_MPTCP);

No need to change anything else in the application.

This patch allows MPTCP protocol in the Socket unit configuration. So
now, a <unit>.socket can contain this to use MPTCP instead of TCP:

  [Socket]
  SocketProtocol=mptcp

MPTCP support has been allowed similarly to what has been already done
to allow SCTP: just one line in core/socket.c, a very simple addition
thanks to the flexible architecture already in place.

On top of that, IPPROTO_MPTCP has also been added in the list of allowed
protocols in two other places, and in the doc. It has also been added to
the missing_network.h file, for systems with an old libc -- note that it
was also required to include <netinet/in.h> in this file to avoid
redefinition errors.

Link: https://www.rfc-editor.org/rfc/rfc8684.html [1]
Link: https://www.mptcp.dev [2]

src/basic/missing_network.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <netinet/in.h>
+
 /* linux/in6.h or netinet/in.h */
 #ifndef IPV6_UNICAST_IF
 #define IPV6_UNICAST_IF 76
@@ -11,6 +13,11 @@
 #define IPV6_TRANSPARENT 75
 #endif
 
+/* linux/in.h or netinet/in.h */
+#ifndef IPPROTO_MPTCP
+#define IPPROTO_MPTCP 262
+#endif
+
 /* Not exposed but defined at include/net/ip.h */
 #ifndef IPV4_MIN_MTU
 #define IPV4_MIN_MTU 68