Use xdr_alloc and xdr_free within ptuser

Use xdr_alloc and xdr_free within ptuser

If the memory allocation routines for libafsauthent and libafsrpc
differ, callers of the ptuser routines in libafsauthent will crash.
Use xdr_alloc and xdr_free in ptuser to ensure that the memory
allocation routines are always consistent.

LICENSE MIT

Change-Id: I20fa1e1fc0677917e47e2ed9f0eaec83f23b699b
Change-Id: Id31bf20b482e9502a5af79f4d86319fe530aa5a5
Reviewed-on: http://gerrit.openafs.org/319
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>