Add rxgk boilerplate

Just the skeleton of what needs to be there.  The actual import is split
over multiple commits, to make the reviewer's burden more manageable.

Error table, protocol description, and stubs for the security object
routines, with header to declare them.

The public header rxgk.h currently only contains a few typedefs and the
NewSecurityObject prototypes, and includes the RPC interface and com_err
code headers.

Change-Id: I7893f78119bb4aef12112cc1e51e1ec69de326c2
Reviewed-on: http://gerrit.openafs.org/10562
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>