rewrite sort_UE for UL and DL and adapt the rest to the change

The previous version did not seem to work properly with
several UEs. I too often ended up with a list 'next' where
next[i] = i, leading to an infinite loop, crashing the
processing.

The sorting functions were hard to understand and too slow
anyway (O(n*n)).

We now use qsort_r and comparison functions, that should
define the same order as the previous version.

One important point is that the "next" lists and the "head"
variable are now considered invalid before calling sort_UE.

So all the code (coming before sort_UE) that looped using those
variables has been rewritten to loop over all possible UEs and
the first instruction of the loop is to exit the loop if the UE
is not 'active'. There is room for improvement here. We will
reintroduce a list of some kind at some point. Let's first
have something that works and then optimize. Today NUMBER_OF_UE_MAX
is 16, that's not a big deal.

Consequently, the add_new_ue and rrc_mac_remove_ue have also
been rewritten, hopefully for the better.

This commit is not a basic work and may introduce some
problems.