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.

Change-Id: I0cee370e25d10b1ca4627832097a9a9f1e83b4c2
Reviewed-on: http://gerrit.openafs.org/4748
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>