q3map2: disable magic path guess

There is an implicit behavior triggered by the lack of basepath that looks for a basepath pattern in every q3map2 argument, this behavior is tricky and can bite hard in the ass. It was recently added to NetRadiant an option to not load the engine path (to allow people to load git repositories without messing with the stable installed game for example), that option is translated to q3map2, triggering the basepath divination. This -fs_nomagicpath option is implied when used -fs_nobasepath but exists as a separate option to call it explicitly in other case (for example if no basepath is set but -fs_nobasepath is not used).