added some optimisation for the case where the number of open files is

added some optimisation for the case where the number of open files is
very large. files.c now promotes a files_struct to the top of the list
if it is used when it is more than 10 elements from the top.

also moved common linked list code for the 5 sets of linked lists that
I've created over the past few days into dlinklist.h (I've explained
to Chris why I didn't use the ubiqx code)