[PATCH 0 of 4] Add Multipath TCP (MPTCP) support

[Anthony Doeraene]

Add Multipath TCP (MPTCP) support

Multipath TCP (MPTCP), standardized in RFC8684 [1], is a TCP extension
that enables a TCP connection to use different paths.

Multipath TCP has been used for several use cases. On smartphones, MPTCP
enables seamless handovers between cellular and Wi-Fi networks while
preserving Established connections. This use-case is what pushed Apple
to use MPTCP since 2013 in multiple applications [2]. On dual-stack
hosts, Multipath TCP enables the TCP connection to automatically use the
best performing path, either IPv4 or IPv6. If one path fails, MPTCP
automatically uses the other path.

The 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 [3].
To use it on Linux, an application must explicitly enable it when
creating the socket. No need to change anything else in the application.

Even if MPTCP is supported by different OS, only Linux supports the
`IPPROTO_MPTCP` protocol, which is why this feature is currently
limited to Linux only.

This is a new version of the series sent by Maxime Dourov a few
months ago. He was no longer able to continue to look at that. I
hope I applied all comment sent by Maxim Dounin in [4].

ChangeLog:
    - v2:
      - Patch 1: 
        - no seamless fallback to the default protocol

      - Patch 2: 
        - Rename flag to multipath instead of mptcp as suggested.
        - Replace the check to include the multipath flag to use #ifdef
          IPPROTO_MPTCP, instead of #if NGX_LINUX, allowing any platform
          supporting MPTCP to enable this option
        - Delete redefinition of IPPROTO_MPTCP if not defined

      - Patch 3: 
        - Rename flag to multipath instead of mptcp as suggested.
        - Add missing check #ifdef IPPROTO_MPTCP, to ensure that MPTCP
          is supported
        - Delete redefinition of IPPROTO_MPTCP if not defined

      - Patch 4: 
        - Rename flag to multipath instead of mptcp as suggested.
        - Replace the check to include the multipath flag to use #ifdef
          IPPROTO_MPTCP, instead of #if NGX_LINUX, allowing any platform
          supporting MPTCP to enable this option
        - Delete redefinition of IPPROTO_MPTCP if not defined

Co-developed-by: Maxime Dourov <mux99 at live.be>

Link: https://www.rfc-editor.org/rfc/rfc8684.html [1]
Link: https://www.tessares.net/apples-mptcp-story-so-far/ [2]
Link: https://www.mptcp.dev [3]
Link: https://freenginx.org/pipermail/nginx-devel/2024-May/000300.html [4]