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.

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>
(cherry picked from commit 50b6a116a1c412d0e6d7442d13d6e92c9dbb35ee)

Change-Id: Iaa340515bf36d249d4f2160b55c3fbb267bb90ee
Reviewed-on: http://gerrit.openafs.org/3825
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>