OPENAFS-SA-2018-001 backup: use authenticated connection to butc

OPENAFS-SA-2018-001 backup: use authenticated connection to butc

Use the standard routine to pick a client security object, instead of
always assuming rxnull.  Respect -localauth as well as being able to
use the current user's tokens, but also provide a -nobutcauth argument
to fall back to the historical rxnull behavior (but only for the connections
to butc; vldb and budb connections are not affected).

Change-Id: Ibf8ebe5521bee8d0f7162527e26bc5541d07910d