Username lookup needs a tuneup

[mvpel]

When submitting a Caffe run to an HTCondor scheduler, the job's environment is not, by default, delivered to the job environment.

This meant that the "USER" variable was empty in the exec node, and so glog named the log file with "invalid-user" in the log name created by Caffe.

This section of code needs a little TLC - currently it merely checks for a USERNAME variable on Windows and USER on Linux. While those are quick and self-contained, it would be best - if that failed - to attempt to look up the username with things like GetUserNameEx or getpwuid on the uid number.

Alternatively, instead of putting in "invalid-user" when a name is not found, putting "uid12345" would have the benefit of preventing log name collisions between different users.

In my case it's an easy workaround, I just expressly deliver the submitter's USER environment variable to the cluster job, so this can be considered a low-priority, low-urgency issue.

[DariuszOstolski]

I've fixed this issue on Linux in #245