bug 518641 - fix the "unify" script to properly handle Java .class files. r=bsmedberg Java .class files and Mach-O fat binaries use the same 4-byte magic number at the start of the file. 'unify' uses the magic number to detect Mach-O binaries, so it chokes on Java .class files. This change makes us use the same heuristic as file(1), which is to check the second 4 bytes in the file. Java class files put a version number there, and Mach-O fat binaries put the number of contained architectures there. Conveniently, Mach defines only 18 architectures, and Java's lowest shipping version number is 43, so there's no overlap in valid values.