Windows: replace strdup with xdr_alloc in callback processing

Windows: replace strdup with xdr_alloc in callback processing

The CRT allocator cannot be used for memory that will be freed
by afsrpc.dll.  Use xdr_alloc() instead.

Reviewed-on: http://gerrit.openafs.org/6439
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
(cherry picked from commit 19f1e0f7cf40c3da1d49c789358bd814f7e3b3eb)

Change-Id: Ie2359b4df312d090f0579ecd6d50ee197b51d68a
Reviewed-on: http://gerrit.openafs.org/6820
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>