Avoid calling user.Current() on windows

[bentekkie]

Use the current process token to look up the user's name on Windows.

This is more reliable than using the USER or USERNAME environment variables, which are not always set, or might be overridden by the user accidentally or
maliciously.

cl/649607112 (google-internal)

[philwo]

Thank you for the suggestion to apply my fix that I sent for reclient to glog directly instead. I think that's indeed the better solution!

As mentioned on #69, I think this fix is better than our internal one and should be merged preferably. Unfortunately I can't LGTM this PR.

For context, here's my commit message from the original PR:

"Unfortunately the Go standard library does not provide a lightweight
method to get just the short username of the current user. Instead, the
provided method user.Current() gathers extra information such as the
home directory and the full (display) name of the user as well.

The latter can be a very expensive operation on corporate Windows, as it
involves making a network call to Active Directory. On my workstation
this takes ~500ms, which directly results in a ~500ms longer reproxy
startup time. The even worse case is when Active Directory cannot be
reached for some reason, resulting in a ~2 minute startup delay for me.

By replacing the call to user.Current() on Windows with just the
minimum calls necessary to get the information we want (short username),
we can avoid all of that. This change reduces the overhead to 10ms in
all cases on Windows."

Here's the upstream bug I filed: http://go.dev/issue/68312

[philwo]

@stapelberg Could you help us out with a review here? 😊