crypto/tls: accept HelloRetryRequest messages with only a cookie

Clients have to reject any HelloRetryRequest message that doesn't lead
to a change in the ClientHello. Instead, we were rejecting any HRR that
didn't select an alternative group, even if it sent a cookie, which
would change the CH.

The good news is that I know of no TLS servers that use or need HRRs
exclusively for cookies (which are mostly useful in DTLS as a way to
verify the source address). The bad news is that we poisoned the
ecosystem as Go 1.12 to 1.14 will reject such HRRs. Oops, hopefully no
one needed this.

No tests because neither Go nor s_server support cookies. This would
presumably get covered once we integrate BoGo.

Fixes #30149

Change-Id: I760fb1ded81148ac3096cf201cbc1e941374b83d
Reviewed-on: https://go-review.googlesource.com/c/go/+/231039


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>