ihandle positional read and write

ihandle positional read and write

When available, use POSIX positional read and write
calls in the ihandle package. Originally written
by Derrick Brashear and Andrew Deason.

(cherry picked from commit 335ccb4082657b7d0e4e9af1076356cf115642d2)
Reviewed-on: http://gerrit.openafs.org/1562
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Change-Id: I6616a8f7ba04410c57097fb6fa687258b847ac4e
Reviewed-on: http://gerrit.openafs.org/3884
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>