pakfire.cgi: Implement Post/Redirect/Get pattern

pakfire.cgi: Implement Post/Redirect/Get pattern

Refreshing the Pakfire page may cause a command to be
executed multiple times and induce odd errors.

This patch implements a HTTP 303 redirect after form processing,
which causes the browser to discard the POST form data.
Navigating backward or reloading the page now does not trigger
multiple executions anymore.

Fixes: #12781
Signed-off-by: Leo-Andres Hofmann <hofmann@leo-andres.de>
Acked-by: Peter Müller <peter.muelle@ipfire.org>