Add a new logging_file implementation

Add a new logging_file implementation

This adds a new logging_file subclass of ui_file.  This new subclass
handles the details of logging, by consulting the relevant globals.

I think a dependency on globals is warranted here, because the logging
settings themselves are global.

The idea of this approach is that rather than modifying the output
pipeline in response to logging commands, a logging_file will simply
always be in the pipeline, and will then react to the appropriate
settings.  ("Appropriate" because there are tests that the logger
doesn't immediately react to changes, so it captures settings at the
moment logging starts.)

The new code isn't actually used yet -- nothing in this patch
constructs a logging_file.  It's separate for easier review.

Approved-By: Andrew Burgess <aburgess@redhat.com>