There was a problem with several UEs in the wireshark log. It seems that
wireshark needs the UE id otherwise it gets confused and sees retransmissions
where it's just another UE connecting and doing the same as a previous
UE.

Let's put kind of a UE id. Each RNTI seen by macpdu2wireshark gets an UE id
assigned the first time it's seen in a trace.

This is a bit hackish but seems to be fine.

We could have added UE_id in all the traces, that is change the T() calls
in the softmodem. It was less work to do it as it is, ie. only touch
macpdu2wireshark.c.

The RAR uses the same UE id all the time, may be a problem. To be refined
if needed.