rx: Add struct rx_identity

rx: Add struct rx_identity

Add the rx_identity structure which can be used by rx security
layers to store and return identity information to applications.

A number of helper functions for manipulating rx_identity structures
are also provided.

Change-Id: I64ae2b62a4bc8a401c1ac877f4662c66a39247f8
Reviewed-on: http://gerrit.openafs.org/2827
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>