windows: add iphlpapi functions for change notifications

[database64128]

The NotifyIpInterfaceChange and NotifyUnicastIpAddressChange functions
register a user-defined callback function for receiving network
interface and IP address change notifications. The GetIfEntry2Ex and
GetUnicastIpAddressEntry functions can be called to retrieve complete
information about the changed interface or address.

The CancelMibChangeNotify2 function deregisters for change
notifications.

[gopherbot]

This PR (HEAD: f2bb83c) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/sys/+/603755.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/603755.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Ian Chen:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/603755.
After addressing review feedback, remember to publish your drafts!