Allow the user to define a custom mode when fopen-ing a log file

[phrfpeixoto]

I've included changes to both allow custom fopen modes, as well as change the visibility of the helper functions on this class.
I've made them final to keep the "do not touch" aspect, while still allowing subclasses to use them.

[stof]

with the new argument, do you actually need to extend the class and call those protected functions ? If yes, why ? If no, I would not change their visibility as making them protected means they become part of the API that is covered by semver (and so they cannot be refactored to change their signature or be removed if the code does not need them anymore)

[phrfpeixoto]

It doesn't. I've added that specifically to bring that discussion:

• If I change the method visibility I can override the class and solve my problem, so I don't need the extra parameter.
• If I add the extra parameter, I no longer need to override the class, so I don't need the method visibility.

Whichever change is less disruptive for the project is OK for me.

IMO, it's likely that you'll want to change the class constructor's signature (other than those helper functions) as time passes, so if you're worried about semver, it would be better to expose those helper methods (they are final, after all).

[phrfpeixoto]

@stof Let me know what changes you would prefer.