TST: Issue #2806 added test forfromfile/tofile with large arrays
FIX: fixing handling tofile/fromfile for large arrays Issue #2806
implemented NumPyOS_fread that would standardise all fread calls as well
as NumPyOS_fwrite to unify the fwrites.

Also made multiarray/ctors.c and multiarray/convert.c to use these new
functions.

see discussion in issue #2806
Unified fread/fwrite limits for NumPyOS_fread and NumPyOS_fwrite
BLD: Moved NumPyOS_fread and NumPyOS_fwrite to numpyos.c rather than npy_os.h. This is where it was meant to go in the first place, I just misunderstood @pv.
FIX: Fix incorrect calculation of bytes read/written in
NumPyOS_fread/NumPyOs_fwrite as pointed out by
@seberg

Also changed the comment style to be consistent with the rest of numpy.