This tracer allows the connection of several tracers to one tracee.

To use the multi tracer:
- run the tracee as usual, eg. ./lte-softmodem -O xxxx.conf --T_stdout 0
- run the multi tracer: ./multi -d ../T_messages.txt
- connect other tracers to the multi tracer:
   - ./enb -d ../T_messages.txt -p 2022
   - ./textlog -d ../T_messages.txt -p 2022
   - etc.

By default the multi tracer connects to the tracee to the port 2021,
as for any other tracer.

Then it accepts connections from other tracers on the port 2022.

This can be changed with parameters on the command line.

The tracee and the other tracers can be stopped/started at any time.

enb.c will then try to reconnect.

as special events.

If we store all received events to a file, this dump is now
self-contained. Helps to analyze traces.

Some functions have also been factorized in the process, too much
duplication.

(Also tracer/Makefile was forgotten in previous commit, included
in this one...)