add MPTCPv1 support

[geliangtang]

The Multipath TCP (MPTCP) protocol (v1 / RFC 8684) has been added in the upstream Linux kernel since v5.6.

MPTCP is strongly tied to TCP, and the kernel APIs are almost the same. The only required dependency is the 'IPPROTO_MPTCP' protocol number definition, which should be provided by the netinet/in.h header if it is recent enough.

This patch adds a new flag '-m' or '--multipath' to support MPTCPv1. It can be used like this:

iperf3 -m -s
iperf3 -m -c 127.0.0.1

There is no need to check for IPPROTO_MPTCP support in configure.ac at build time, it is at runtime we will see if the kernel being use supports or not MPTCP.

If IPPROTO_MPTCP is not supported by the kernel being tested, it is normal to fail because the feature is not available and the user explicitly asked to use MPTCP.

Co-developed-by: Paolo Abeni [email protected]

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