* man/C/dpkg-scanpackages.1: Document new dpkg-scanpackages

* man/C/dpkg-scanpackages.1: Document new dpkg-scanpackages
-m option.
* scripts/dpkg-scanpackages.pl: Rewrite the script to support
multiple versions of packages in a single Packages file;
use Getopt::Long instead of attempting to parse the command line
ourselves and doing it badly;
get rid of unecessary hashes and arrays that aren't used at all;
output help when given the --help/-h/-? options
Closes: #229589, #319541