Fix gtx_create calls to all share a prototype

Fix gtx_create calls to all share a prototype

All of the different _create calls need to share a prototype, so
function pointers can be stuffed into a structure. Use an anonymous
pointer so that we can acheive this.

Reviewed-on: http://gerrit.openafs.org/187
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>