xdrproc_t functions take a caddr_t, not caddr_t*

xdrproc_t functions take a caddr_t, not caddr_t*

Callers of xdrproc_t functions give the function a foo* as the second
argument, not a foo**, and the xdrproc_t functions themselves expect
this. Make the xdrproc_t typedef accurately reflect that, thereby
ridding us of some warnings.

Reviewed-on: http://gerrit.openafs.org/603
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>