libpar2 Patch: Bugfix and Enhancement

This is the patch from NZBGet 9.1, won't be added upstream as upstream hasn't seen any activity in years.

Bugfixes and ability to cancel par checking, needed for new NZBGet.

Tried to do this as a submodule (can be found [here](https://gist.github.com/4576894)), but I couldn't get libpar2 to install to the Cellar nicely (installed into Cellar/__UNKNOWN__/0.2), and obviously the way the file currently is, the compiled libpar2 gets deleted when NZBGet finishes installing, so NZBGet won't run.