support reading metadata from compressed files

support reading metadata from compressed files

The raw format provides very little metadata.  Allow filters to pass
back state that it knows about.  With gzip, we know the original file
name, mtime, and file size.  For now, we only pull out the first two
as those are available in the file header.  The latter is in the file
trailer, so we'll have to add support for that later (if we can seek
the input).