Function do_pset

* do_pset * * Performs the assignment "param = value", where value could be NULL; * for some params that has an effect such as inversion, for others * it does nothing. * * Adjusts the state of the formatting options at *popt. (In practice that * is always pset.popt, but maybe someday it could be different.) * * If successful and quiet is false, then invokes printPsetInfo() to report * t