Forbid a server from negotiating both ALPN and NPN.

If the two extensions select different next protocols (quite possible since one
is server-selected and the other is client-selected), things will break. This
matches the behavior of NSS (Firefox) and Go.

Change-Id: Ie1da97bf062b91a370c85c12bc61423220a22f36
Reviewed-on: https://boringssl-review.googlesource.com/5780
Reviewed-by: Adam Langley <agl@google.com>