cmd/go: replace -getmode with -mod, $GOPROXY

The old -getmode flag had two settings:
-getmode=local meant don't download from the network.
-getmode=vendor meant only use the vendor directory.

The new -mod flag has two settings:
-mod=readonly means refuse to automatically update go.mod (mainly for CI testing).
-mod=vendor means only use the vendor directory.

The old GOPROXY variable had two settings:
a proxy URL or else the empty string (direct connect).

The new GOPROXY variable has three settings:
a proxy URL, the string "off" (no network use allowed),
or else the empty string or the explicit string "direct" (direct connection).
We anticipate allow a comma-separated sequence in a future release,
so commas are disallowed entirely right now.

Fixes #24666.
Fixes #26586.
Fixes #26370.
Fixes #26361.

Change-Id: If2601a16b09f04800f666938c071fc053b4c3f9c
Reviewed-on: https://go-review.googlesource.com/126696


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>