grdfilter: Fix the long names for arguments of the -N option

[seisman]

The current long names are not readable. I've fixed them to match the source codes below:

gmt/src/grdfilter.c

Lines 867 to 883 in 812e370

	case 'N': /* Treatment of NaNs */
	n_errors += gmt_M_repeated_module_option (API, Ctrl->N.active);
	switch (opt->arg[0]) {
	case 'i':
	Ctrl->N.mode = NAN_IGNORE; /* Default */
	break;
	case 'r':
	Ctrl->N.mode = NAN_REPLACE; /* Replace */
	break;
	case 'p':
	Ctrl->N.mode = NAN_PRESERVE; /* Preserve */
	break;
	default:
	GMT_Report (API, GMT_MSG_ERROR, "Expected -Ni|p|r\n");
	n_errors++;
	break;
	}

[joa-quim]

These must have escaped me in first proposal. They were really bad.