Tidy up uio_t meaning

Tidy up uio_t meaning

On IRIX, uio_t is typedef'd to "struct uio".
On Darwin, uio_t is typedef'd to "struct uio *".

Reduce the confusion by just not using "uio_t" in places where it
isn't being defined for us, and avoiding it completely in cross-platform
code.

Reviewed-on: http://gerrit.openafs.org/4748
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit ca465e432d67b067fc3854781b8702aaff577cc7)

Change-Id: Id37d11e72d196f5008e9843d4f04c492a4017e72
Reviewed-on: http://gerrit.openafs.org/5779
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>