debuginfod: Use the debuginfod-size response header

debuginfod: Use the debuginfod-size response header

In some cases the content-length header may not be available in order
to pass to a progressfn.  If content-length isn't available then attempt
to get the size of the download from the debuginfod-size header instead.

It should be mentioned that if a compressed file (ex. gzip) is being
transferred, the actual transfer length will be less than debuginfod-size.
In this case debuginfod-size is a best-guess upper bound on the size of
the transfer.

Signed-off-by: Aaron Merey <amerey@redhat.com>