Improve identification of the temp directory and location of main log file

The temp dir is now in the form
                 <temp>/<user>/stressProof-<pid>

explicitely notified when starting the test.
The main log file (the one from the stressProof binary is now located under the
same test directory and named 'main.log'. E.g.

$ ./stressProof
******************************************************************
*  Starting  P R O O F - S T R E S S  suite                      *
******************************************************************
*  Temp dir for this run: /tmp/ganis/stressProof-31556
******************************************************************
*  Log file: /tmp/ganis/stressProof-31556/main.log
******************************************************************
...