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.

The support first appeared in OS X Mavericks 10.9 and documented since
OS X Yosemite 10.10.

It has a subtle implementation difference from other operating systems
in that the TCP_KEEPALIVE socket option (used in place of TCP_KEEPIDLE)
isn't inherited from a listening socket to an accepted socket.

An apparent reason for this behaviour is that it might be preserved for
the sake of backward compatibility.  The TCP_KEEPALIVE socket option is
not inherited since appearance in OS X Panther 10.3, which long predates
two other TCP_KEEPINTVL and TCP_KEEPCNT socket options.

Thanks to Andy Pan for initial work.

MTU selection starts by doubling the initial MTU until the first failure.
Then binary search is used to find the path MTU.

Previously, ngx_udp_rbtree_insert_value() was used for plain UDP and
ngx_quic_rbtree_insert_value() was used for QUIC.  Because of this it was
impossible to initialize connection tree in ngx_create_listening() since
this function is not aware what kind of listening it creates.

Now ngx_udp_rbtree_insert_value() is used for both QUIC and UDP.  To make
is possible, a generic key field is added to ngx_udp_connection_t.  It keeps
client address for UDP and connection ID for QUIC.

As per RFC 9000, Section 14:

  UDP datagrams MUST NOT be fragmented at the IP layer.

Previously, QUIC used the existing UDP framework, which was created for UDP in
Stream.  However the way QUIC connections are created and looked up is different
from the way UDP connections in Stream are created and looked up.  Now these
two implementations are decoupled.

Previously it had -1 as fd.  This fixes proxying, which relies on downstream
connection having a real fd.  Also, this reduces diff to the default branch for
ngx_close_connection().