We take a best-effort approach since information for these fields
are not well supported on all platforms.

user.LookupId+user.LookupGroupId is currently 15x slower than os.Stat.
For performance reasons, we perpetually cache username and groupname
with a sync.Map. As a result, this function will not be updated whenever
the user or group names are renamed in the OS. However, this is a better
situation than before, where those fields were not populated at all.

Change-Id: I3cec8291aed7675dea89ee1cbda92bd493c8831f
Reviewed-on: https://go-review.googlesource.com/59531
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>