[netcdf-hdf] using POSIX calls when configured for MPI-IO

Hi.

Bet you're going to get tired of me soon!

I'd really like to see all I/O calls using MPI-IO instead of posix
fread (as in nc_check_for_hdf5).   The HDF5 guys might recognize that
I've bugged them about this last year :>

ROMIO can take a file system prefix to override automatic file system
detection.  Such a prefix could be "testfs:" for our tracing file
system driver (does no I/O, but logs which processes wrote how much)
or it could be "pvfs2:" if we want to acess this userspace file system
and do not happen to have the kernel component running.

So for example, in nc_check_for_hdf5, instead of using fread, maybe
use MPI_File_open and MPI_File_read when examining the magic number,
and USE_PARALLEL_MPIIO is defined.  Likewise, nc4_create_file,
and nc_get_file_version use POSIX calls.  

Thanks
==rob

-- 
Rob Latham
Mathematics and Computer Science Division    A215 0178 EA2D B059 8CDF
Argonne National Lab, IL USA                 B29D F333 664A 4280 315B