seekable zip backend: Accept files with valid ZIP64 EOCD headers
Previously, a ZIP64 file needed to have a valid classic EOCD header.
This is not the case at least for archives generated by PHPZipStreamer.
This commit allows for a bit more than necessary: While PHPZipStreamer
sets the classic EOCD header to all -1's, the bidding function now
succeeds also if the EOCD header values have arbitrary values, as long
as the ZIP64 EOCD header seems plausible. This simplifies the logic and
should not do any harm in practice.