The "multipath" parameter of the "listen" directive.

The "multipath" parameter of the "listen" directive.

When configured, it enables Multipath TCP support on a listen socket.
As of now it works on Linux starting with Linux 5.6 and glibc 2.32,
where it is enabled with an IPPROTO_MPTCP socket(2) protocol.

To avoid EADDRINUSE errors in bind() and listen() when transitioning
between sockets with different protocols, SO_REUSEPORT is set on both
sockets.  See f7f1607bf for potential implications.

Based on previous work by Maxime Dourov and Anthony Doeraene.