Fix warnings:  mismatched signs, unused variables.
Fix mismatched sign warning
Break Zip compatibility test into 5 separate tests.
Set compiler flags for Clang.
Out-of-range seeks should fail, not truncate the seek.
Fix all current tests:
 * When searching for start of Central directory, DTRT if there are no entries.
 * Find end-of-archive marker when it is exactly at end of file
 * Recognize zip64 central directory locator (not yet fully parsed)
 * Don't return entry size if length-at-end, even if size looks reasonable
Adjust expected permissions; zip code has slightly tightened
the default permissions used when extracting files.