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* Negate offset when ZIP_SOURCE_SEEK from SEEK_END 

custom_zip_source_callback from the zip library where the cmd is ZIP_SOURCE_SEEK and whence is SEEK_END specifies a negative offset.  pImportStream takes an unsigned positive value to indicate the distance to seek back.  The value must be negated between the two APIs.

* Assume unsigned argument for seekFromEnd

The API for CImportStream and CExportStream only allows unsigned input.
This commit handles this consistently in all seekFromEnd-functions
and adds some exceptions in custom_zip_source_callback.

* Support zip seeking backward from the current position.

* Update version