report current read offset in zip source
zip_source_tell() returns the current read
offset for source. The return value can be passed to
zip_source_seek(3) with whence set to
SEEK_SET to return to the same location in the source.
Upon successful completion the current read offset is returned. Otherwise, -1 is
returned and the error information in source is set to
indicate the error.
zip_source_tell() was added in libzip 1.0.