net: check for MPTCP in DialTCP and ListenTCP

[matttbe]

Setting GODEBUG=multipathtcp= [1] has no effects on apps using
ListenTCP or DialTCP directly.

According to the documentation, these functions are supposed to act like
Listen and Dial respectively:

ListenTCP acts like Listen for TCP networks.
DialTCP acts like Dial for TCP networks.

So when reading this, I think we should expect GODEBUG=multipathtcp= to
act on these functions as well.

Also, since #69016, MPTCP is used by default (if supported) with TCP
listeners. Similarly, when ListenTCP is used directly, MPTCP is
unexpectedly not used. It is strange to have a different behaviour.

So now, ListenTCP and DialTCP also check for MPTCP. Those are the exact
same checks that are done in dial.go, see Listen and dialSingle.

[1] https://pkg.go.dev/net#Dialer.SetMultipathTCP

Fixes #70500

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