change checking for size of dim/attr/header limit

[wkliao]

The maximal header extent for CDF 1 and 2 should be NC_MAX_INT , i,e,
the maximum value of a signed 4-byte integer, 2^31-1. Same for the
dimension size. This is because NetCDF file format defines them as
non-negative signed integers, OFFSET and NON_NEG respectively.

NetCDF file format specification:

netcdf_file  = header data
header       = magic numrecs dim_list gatt_list var_list
dim_list     = ABSENT | NC_DIMENSION nelems [dim ...]
dim          = name dim_length
var          = name nelems [dimid ...] vatt_list nc_type vsize begin
vsize        = NON_NEG
dim_length   = NON_NEG 
NON_NEG      = <non-negative INT>
begin        = OFFSET
OFFSET       = <non-negative INT> | <non-negative INT64>
INT          = <32-bit signed integer, Bigendian, two's complement>

• begin is the file extent, i.e. starting file offset of the data section.
• dim_length is the dimension size.

Note for variable size, the maximal is
2^31-3 for CDF-1
2^32-3 for CDF-2
2^63-3 for CDF-5
Variable size is calculated internally in PnetCDF. Therefore, the
variable size is limited to the space available in bytes. -3 is
due to the 4-byte upward alignment.
Although vsize is defined and stored in the file header, it is marked as a redundant item and is not used in PnetCDF.