Lack of thread guards in DiskFile

[animetosho]

The DiskFile class uses serr to output errors. This class is used in a number of multi-threaded places in Par2Repairer (e.g. here), but there are no guards around serr usage.

Although this is only used for outputting errors, it still would make sense to mark these as critical sections.

Also worth pointing out that sout is never used in the class and could be removed.
It might also be worth considering exporting all the logging functions to some abstraction layer, as it could help with libpar2 usage where applications don't want output on stdout/err.