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).
(cherry picked from commit
345ee34236c08a0a2fb3fff016edfa18c7af4b0a)
(cherry picked from commit
ed217df4b23e111d4b12e7236bdf6f8ab5575952)