Add interface to select client security objects

Add interface to select client security objects

Add a pair of interfaces to support the selection of a security
object by the client. The idea of these interfaces is to abstract
out the job of selecting an interface from the client code itself,
and into a common library. This reduces duplicated code, and makes
it easier to add new security objects in the future.

Change-Id: I2bf411e6b56534070c827d009d36fa8a618c4511
Reviewed-on: http://gerrit.openafs.org/1219
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>