net: fixes to dnsReadConfig in dnsconfig_windows.go
- Only search DNS servers for network interfaces with at least one gateway
- Clarify comment on deprecated site local anycast fec0/10 DNS IPv6 addresses
- Minor maintenance: skip not "up" interfaces earlier in outer loop
Change-Id: I98ca7b81d3d51e6aa6bfa4a10dcd651305a843df
GitHub-Last-Rev:
3b358c7e3f89971d069286f997dc19e092ec8f08
GitHub-Pull-Request: golang/go#64441
Reviewed-on: https://go-review.googlesource.com/c/go/+/545775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>