os/user: faster user lookup on Windows

os/user: faster user lookup on Windows

Trying to lookup user's display name with directory services can
take several seconds when user's computer is not in a domain.
As a workaround, check if computer is joined in a domain first,
and don't use directory services if it is not.
Additionally, don't leak tokens in user.Current().
Fixes #5298.

R=golang-dev, bradfitz, alex.brainman, lucio.dere
CC=golang-dev
https://golang.org/cl/8541047