windows: add FlushConsoleInputBuffer and GetNumberOfConsoleInputEvents

This adds syscall wrappers for FlushConsoleInputBuffer and GetNumberOfConsoleInputEvents on Windows.

Change-Id: I2365aebc42a57f83cfc951e10520270e1f3e0606
GitHub-Last-Rev: 1f711a5f3c2b09b6bc74160f0fc7c54a0247be14
GitHub-Pull-Request: golang/sys#264
Reviewed-on: https://go-review.googlesource.com/c/sys/+/704715
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>