fix endianness problem with the tail of the signature comment

fix endianness problem with the tail of the signature comment

The two 0xff bytes were intended to easily distinguish files with
whole file signatures from those without, but I got the endianness
backwards.  Go ahead and fix that, as long as I'm making changes to
the verifier anyway.

Check for a signature that includes the sequence 0x50 0x4b 0x05 0x06,
which looks to minzip like the start of the EOCD block.