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