Recently I needed to be able to recognize multiple use of an option when 
using getopt-long. For example, the gcc "-L" option can be specified 
multiple times for various paths. The option-ref helper function only 
seems to get the most recent use of an option using assq. I couldn't see 
any other way in the getopt-long API to do it, so added a helper 
function option-ref-list (see patch vs stable-2.0).