crypto/tls: ensure the server picked an advertised ALPN protocol
This is a SHALL in RFC 7301, Section 3.2.
Also some more cleanup after NPN, which worked the other way around
(with the possibility that the client could pick a protocol the server
did not suggest).
Change-Id: I83cc43ca1b3c686dfece8315436441c077065d82
Reviewed-on: https://go-review.googlesource.com/c/go/+/239748
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Roland Shoemaker <roland@golang.org> Reviewed-by: Roland Shoemaker <roland@golang.org>