Packages files now contain `size' and `md5sum'

[Ian Jackson <ijackson@chiark.chu.cam.ac.uk>]

I've updated mkpackages so that it puts the size in bytes and MD5
checksum of the files in the Packages files.

I didn't put them in the same field because it seemed silly for
programs and humans to have to parse the contents of a field into two
essentially unrelated pieces of information, and because I couldn't
think of a good field name :-).

The actual Packages files are being updated as I write this.

I've also changed mkpackages to leave out any empty fields that might
have been in the original packages' control files.

Ian.