Numerous improvement to extract-deb-pkg, in no particular order:

 * Use bash
 * More strict error checking (set -u -o pipefail)
 * Print error message if target dir exists
 * Check number of arguments, provide usage info
 * Separate mirror to a variable for easier changing
 * Use mirrors.kernel.org
 * Use https and check cert with curl
 * Make curl silent
 * Use zgrep instead of gunzip | grep
 * Error out with a message in case more than 1 package was found
 * Do not create a target directory before we need it
 * Fix shellcheck warnings (way too many to mention)
 * ...
 * PROFIT!!!111
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>