windows: add SO_SNDTIMEO constant for socket options

Add SO_SNDTIMEO to complement the previously added SO_RCVTIMEO.

Change-Id: Ief9370900b5b057f3f03384d5219f1bf836cf0ea
GitHub-Last-Rev: 75b866b04ef219c599dfce93d12519b32a31b0aa
GitHub-Pull-Request: golang/sys#286
Reviewed-on: https://go-review.googlesource.com/c/sys/+/819680
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>