windows: add constants for PMTUD socket options

Related documentation:

- https://learn.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options
- https://learn.microsoft.com/en-us/windows/win32/winsock/ipproto-ipv6-socket-options

Change-Id: I21b23ca815d1d8135ce5724115b9ca23819ea10a
GitHub-Last-Rev: 9054c5c79068da8d2583b23dca9464293c36901a
GitHub-Pull-Request: golang/sys#245
Reviewed-on: https://go-review.googlesource.com/c/sys/+/654495
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>