The renegotiation extension was introduced[1] due to an attack by Ray in
which a client's handshake was spliced into a connection that was
renegotiating, thus giving an attacker the ability to inject an
arbitary prefix into the connection.

Go has never supported renegotiation as a server and so this attack
doesn't apply. As a client, it's possible that at some point in the
future the population of servers will be sufficiently updated that
it'll be possible to reject connections where the server hasn't
demonstrated that it has been updated to address this problem.

We're not at that point yet, but it's good for Go servers to support
the extension so that it might be possible to do in the future.

[1] https://tools.ietf.org/search/rfc5746

R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/48580043