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*

Reviewed-on: http://gerrit.openafs.org/3744
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 6d2636b886789f2ddc6374c6b7e58ccd28d518aa)

Change-Id: Ib07ea6d1e0dad935717810c1279cc7f38645e229
Reviewed-on: http://gerrit.openafs.org/3863
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>