glog: check that stderr is valid before using it by default (#72)

Windows Services are [spawned without `stdout` and `stderr`](https://learn.microsoft.com/en-us/windows/console/getstdhandle#return-value:~:text=If%20an%20application%20does%20not%20have%20associated%20standard%20handles%2C%20such%20as%20a%20service%20running%20on%20an%20interactive%20desktop%2C%20and%20has%20not%20redirected%20them%2C%20the%20return%20value%20is%20NULL.), so any attempt to use them equates to referencing an invalid file `Handle`. `stderrSink` returns the error, which makes `logsink` trigger the termination of the process. The result is that any call to `log.Error` (or `log.Fatal`) from a Windows Service will always crash the program.

This approach checks that `os.Stderr`'s embedded file descriptor (`Handle`, for Windows) is non-NULL once during `init` to determine its validity and if it fails, then never registers `stderrSink` as a logsink.

Disabling based upon whether `os.Stderr` is NULL was chosen because it's precise, doesn't actually influence the result of any output and doesn't require any syscalls, but still detects the case in which `stderr` is unavailable on Windows.

A rejected approach here would've been to swallow the errors from `stderrSink.Emit`, thus sparing us program termination at the hands of `logsink`. I rejected this approach because it felt like it could result in some subtle behavioral changes. For example, invoking `log.Error` after `os.Stderr.Close()` would no longer exit the program and that felt like a non-trivial change (although I can think of no reason why one would desire this behavior).

cl/680817112 (google-internal)