gkeys.log: Isolate file-handler code in the logpath branch

There's no need for this variable if logpath isn't set, so remove the global
variable and external references.

Also downcase it to the more Pythonic 'file_handler'.  PEP8 doesn't even
recognize a 'Sentence_case' naming style [1].

[1]: http://legacy.python.org/dev/peps/pep-0008/#descriptive-naming-styles