shlibafsrpc: Add additional XDR functions

Export the afs_xdr_pointer and afs_xdr_u_char functions so that
shlibafsauthent can make use of them.

Reviewed-on: http://gerrit.openafs.org/7065
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 00992c66972aeab96cca9781b02a780ab49dce23)

Change-Id: I48ef13427c53e8fe3ceca00eb5d9d35c2c4327d7
Reviewed-on: http://gerrit.openafs.org/8809
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>