runtime: make netpollBreak entries identifiable on Windows

runtime: make netpollBreak entries identifiable on Windows

It is currently not possible to distinguish between a netpollBreak
entry and an entry initiated by external WSA operations (as in #58870).

This CL sets a unique completion key when posting the
netpollBreak entry so that it can be identified as such.

Change-Id: I8e74a7ddc607dc215d6ed8c59d5c3cf47ec8dc62
Reviewed-on: https://go-review.googlesource.com/c/go/+/558895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>