track go/issues/56539: net: add support for MPTCP

[komuw]

• net: add support for MPTCP golang/go#56539
• golang/go@383a4e7

In the issue, rsc said;

I am inclined to say that we add MPTCP as an opt-in for a release or two, and then make it opt-out. 
There should be a GODEBUG setting 

We should see if we can send a patch upstream to make it opt-out

[komuw]

The Go v1.21 release notes say;

On Linux, the net package can now use Multipath TCP 
when the kernel supports it. It is not used by default. 
To use Multipath TCP when available on a client, call the `Dialer.SetMultipathTCP` method before calling 
the `Dialer.Dial` or `Dialer.DialContext` methods.

To use Multipath TCP when available on a server, call the `ListenConfig.SetMultipathTCP` method 
before calling the `ListenConfig.Listen` method. 
Specify the network as "tcp" or "tcp4" or "tcp6" as usual.

[komuw]

PR upstream; https://go-review.googlesource.com/c/go/+/563575

That PR is blocked on multipath-tcp/mptcp_net-next#383