better searching for SZARP configurations

+ stem() would transform /opt/szarp/szarp/foo.bar to foo, which would lead to a segfault e.g. in ekstraktor3 if we had both foo and foo.bak directories (configuration foo would be read twice, leading to some memory issues)
+ filename() is the way to go with boost::filesystem::path, it returns the last component of a path (stem() is based on filename() btw)