checking if a variable is a record variable when using PnetCDF option

[wkliao]

For PnetCDF, checking whether a variable is a record variable can just check the
first dimension, because CDF-1,2,5 formats allow only the first dimension to be
unlimited.

scorpio/src/clib/pio_nc.c

Lines 1353 to 1365 in ac679a5

	int *p = (dimidsp) ? dimidsp : tmp_dimidsp;
	int is_rec_var = file->varlist[varid].rec_var;
	for(int i=0; (i<ndims) && (!is_rec_var); i++)
	{
	for(int j=0; (j<file->num_unlim_dimids) && (!is_rec_var); j++)
	{
	if(p[i] == file->unlim_dimids[j])
	{
	is_rec_var = 1;
	}
	}
	}
	file->varlist[varid].rec_var = is_rec_var;

The above code block can be simplified to the following.

int *p = (dimidsp) ? dimidsp : tmp_dimidsp;
file->varlist[varid].rec_var = (ndims > 0 && p[0] == NC_UNLIMITED) ? 1 : 0;