os/user: obtain a user GID on Windows

Add the following helpers in lookup_windows.go:
1) lookupGroupName() is used to obtain the SID of a group based
on name.
2) listGroupsForUsernameAndDomain() uses NetUserGetLocalGroups()
as a WINAPI backend to obtain the list of local groups for this
user.
3) lookupUserPrimaryGroup() is now used to populate the User.Gid
field when looking up a user by name.

Implement listGroups(), lookupGroupId(), lookupGroup() and no longer
return unimplemented errors.

Do not skip Windows User.Gid tests in user_test.go.

Change-Id: I81fd41b406da51f9a4cb24e50d392a333df81141
GitHub-Last-Rev: d1448fd55d6eaa0f41bf347df18b40da06791df1
GitHub-Pull-Request: golang/go#24222
Reviewed-on: https://go-review.googlesource.com/98137


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>