manually incorporate fixes for ave/correlate/long from PR #2066

       v_name = global value calculated by an equal-style variable with name

* zero or more keyword/arg pairs may be appended

* keyword = *type* or *start* or *file* or *overwrite* or *title1* or *title2* or *ncorr* or *p* or *m*

* keyword = *type* or *start* or *file* or *overwrite* or *title1* or *title2* or *ncorr* or *nlen* or *ncount*

  .. parsed-literal::

    if(overwrite) fseek(fp,filepos,SEEK_SET);

    fprintf(fp,"# Timestep: " BIGINT_FORMAT "\n", ntimestep);

    for (unsigned int i=0;i<npcorr;++i) {

      fprintf(fp, "%lg ", t[i]*update->dt);

      fprintf(fp, "%lg ", t[i]*update->dt*nevery);

      for (int j=0;j<npair;++j) {

        fprintf(fp, "%lg ", f[j][i]);

      }