os/user: speed up Current on Windows

os/user: speed up Current on Windows

user.Current is slow on Windows sessions connected to an Active
Directory domain. This is because it uses Windows APIs that do RPC
calls to the domain controller, such as TranslateAccountW and
NetUserGetInfo.

This change speeds up user.Current by using the GetUserNameEx API
instead, which is already optimized for retrieving the current user
name in different formats.

These are the improvements I see with the new implementation:

goos: windows
goarch: amd64
pkg: os/user
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
           │   old.txt   │               new.txt                │
           │   sec/op    │    sec/op     vs base                │
Current-12   501.8µ ± 7%   118.6µ ± 11%  -76.36% (p=0.000 n=10)

           │  old.txt   │              new.txt              │
           │    B/op    │    B/op     vs base               │
Current-12   888.0 ± 0%   832.0 ± 0%  -6.31% (p=0.000 n=10)

           │  old.txt   │              new.txt               │
           │ allocs/op  │ allocs/op   vs base                │
Current-12   15.00 ± 0%   11.00 ± 0%  -26.67% (p=0.000 n=10)

Updates #5298
Fixes #21867
Fixes #68312

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest
Change-Id: I893c5fcca6969050d73a20ed34770846becd5f5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/597255
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>