fix bitslot already in use warning

fix bitslot already in use warning

in e45151b89c64 route.cc was changed to create internal sends
directly with role = Delivery::Aux; and not Delivery::Role (0).
This change was motivated to initialize the panner for Aux-sends
in the Delivery.

Role(0) was used to override bitslot numbering during initial
construction of the object when the state is loaded from XML after
construction.

This patch adds an explicit flag for that.
(The previous Role(0) approach only worked for Aux-Sends
but not Sends, anyway.)