AR: Use manager/subordinate for master/slave.

Address AR feedback:
> Master/slave terminology is obviously being replaced across RV specs -
> in a somewhat haphazard manner as far as what is being used for
> replacements.  The ARC suggests using the following (and will be
> proposing this to be the standard replacement across RV specs):
> Manager/subordinate  (which remains intuitive and happens to maintain
> the M/S starting letters fwiw)