Logfile with more restrictive permissions control

It would be great if we could open files with more restrictive permissions: https://github.com/google/glog/blob/master/src/logging.cc#L900. Opening with `o600` would be safer for daemons logging potentially sensitive WARN/ERROR details.

Something like:

``` cpp
FLAGS_logfile_mode = 0600;
```

Would be :+1: 
