Add rx security index enum

Add rx security index enum

Add a enumerated type for rx security indexes, initially containing
the values reserved for null, kad, gk, and k5. Start to use this type,
and related names, rather than hard values throughout the code.

Change-Id: Ic71e5da28b4270abf7b6688b2c6438f17268f1da
Reviewed-on: http://gerrit.openafs.org/1242
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>