Windows: read and write take void* buffers, open takes a const char*

Windows: read and write take void* buffers, open takes a const char*

nt_read and nt_write were defined to take a char* buffer which was
then cast to a void *.  Meantime every call of OS_READ and
OS_WRITE were casting to a char*.

Equally every call of OS_OPEN was passing down a const char*,
causing warnings.

This checkin fixes this:
 nt_read : char* to void*
 nt_write: char* to const void*
 nt_open char*  to const char*

Change-Id: Id4e138b9d347e1a9f35241e162a105d5f462b168
Reviewed-on: http://gerrit.openafs.org/3744
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>