windows: native versions of ih_pread and ih_pwrite

windows: native versions of ih_pread and ih_pwrite

Separate the windows code out in ihandle.c to reduce dependency on ntops.

As an aid to future threading issues, pass the offset in an OVERLAP
rather than doing a separate SetFilePointerEx.

Change-Id: I225387a574b1301516a9313838bbcb86e9e14b8d
Reviewed-on: http://gerrit.openafs.org/3176
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>