rules.pl: Introduce set_loader mechanism

rules.pl: Introduce set_loader mechanism

This introduces a mechanism where each setname and the function
which is used to proper load this set are stored in a hash.

The load_set function will use this information to call the correct
function.

To register a set loader the register_set_loader function can be used by
specifying the function name and an array of affected set names.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>