net: MPTCP support is not enabled when apps use ListenTCP() or DialTCP() [1.23 backport]

[gopherbot]

@matttbe requested issue #70500 to be considered for backport to the next 1.23 minor release.

Do you think this simple fix preventing GODEBUG=multipathtcp=X to work in some cases could be considered for backport to the >= v1.21 versions?

There is no crash, but also no way to change the behavior with GODEBUG as expected, apart from modifying the app.

The code that is being modified has not changed for at least 5-6 years, the cherry-pick should not be an issue.
On these older versions, MPTCP is not used by default, except if GODEBUG=multipathtcp=1 is set, so the risk is limited.

@gopherbot please consider this for backport.