ishold.m: Recode to follow modern Octave coding conventions. * scripts/plot/ishold.m: Put input validation first. Change error messages to reference bad input variable by name. Don't use unnecessary strcmpi when strcmp will work.

#include "mex.h"

void
mexFunction (int nlhs, mxArray *plhs[],
             int nrhs, const mxArray *prhs[])
{
  mexPrintf ("Hello, World!\n");

  mexPrintf ("I have %d inputs and %d outputs\n", nrhs, nlhs);
}