Use the RX_SECIDX_* enums in more places

Add symbols for security indices 1 (rxvab, unused) and 3 (rxkad with
always-encrypt, maybe-used).

kauth and bubasics defined macros for RX_SCINDEX_*; replace those
with the common core rx enums as well.

Use the global symbols instead of custom defines like BULK_NULL and
SAMPLE_NULL, which can now be removed.

Reformat a comment to match current style, since we're changing it anyway.

Change-Id: I82bbb0016a5c3129dfd18ff7dc77ff7839501ad8
Reviewed-on: http://gerrit.openafs.org/10526
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>