CL 111835 changed Transport stream ID numbering to start at
stream 3 when AllowHTTP is set. This was based on a
misunderstanding:

When a connection upgrades an HTTP/1.1 request to HTTP/2,
the initial HTTP/1.1 request occupies stream 1.
However, Transport does not perform HTTP protocol upgrades.
When using a Transport to send unencrypted HTTP/2 requests,
the entire connection uses HTTP/2, the first request is
sent as HTTP/2, and there is no reason not to use stream 1
for this request.

Starting from stream 3 is mostly harmless,
but ClientConn.idleStateLocked assumes that client streams
start from 1. This causes it to misidentify new single-use
connections as having already sent a request (when AllowHTTP
is set), and therefore not suitable for use.

Revert to always starting stream IDs at 1.

Fixes golang/go#67671

Change-Id: I97c89de4ae49623d916f9dbd200f8252d2fd4247
Reviewed-on: https://go-review.googlesource.com/c/net/+/591275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

The ReadMsgUDP and WriteMsgUDP methods of UDPConn are not implemented
(yet?) on Plan 9. Skip tests which require them.

Fixes golang/go#68288

Change-Id: Ic6c81b19322d589c10b16da61e9b89284294be05
Reviewed-on: https://go-review.googlesource.com/c/net/+/596795
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>

Update golang.org/x dependencies to their latest tagged versions.

Change-Id: Ibb0ab46488252d035430a654eed5dd4caab7509e
Reviewed-on: https://go-review.googlesource.com/c/net/+/596895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>