as this was not included, xbt_dynar_sort never used mergesort ..
we should check if we don't forget it elsewhere
these terrible sort operators were working everywhere except on bsd, where qsort is stricter
avoid potential problems when tracing comms coming from any source,
whithout using status to track its origin.
Remove any reference to mergesort.

* it was in fact never used (see commit 7f6d42b)
* it's not standard
* according to http://www.freebsd.org/cgi/man.cgi?query=mergesort&sektion=3,
  - it may fail
  - it's generally slower than qsort
  - it has requirements on its parameter "size"
  - it requires additional memory