Add a command line switch to set the number of used threads

[jwrona]

The number of threads used in an OpenMP parallel region can be influenced by the OMP_NUM_THREADS environment variable, the num_threads clause and the omp_set_num_threads. The easiest way is to set the OMP_NUM_THREADS env. var. using the MPI process manager, but each MPI process manager uses different way of doing that.
It would be better to handle that in fdistdump by a --treads switch or something like that.

[jwrona]

fefcdf9 adds the --num-threads option