Replace symlink heuristic with a well-defined test.

As per the PKWARE ZIP File Format Specification (6.3.2), this
patch tests the file 'version made by' field for the UNIX (3)
constant, and then interprets the external file attributes accordingly,
assuming an st_mode field in the high 16 bits.
Merge pull request #91 from landonf/master

Fix symlink detection