Clarification of the Format field in control files -- patch for #547272.

Clarification of the Format field in control files -- patch for #547272.

The Format field has a different meaning for Debian source control files and
Debian changes files. This patch :

 - documents the use and syntax of the Format field for these two types
   of control files.

 - Specifies the format 1.8 of Debian changes files, adds descriptions for
   the Checksums-Sha1 and Checksums-Sha256 files, and provides some history
   for the Closes, Changed-By and Maintainer fields.