Move contrib/seg to only use V1 calling conventions.

A later commit will remove V0 support.

Author: Andres Freund, with contributions by Craig Ringer
Reviewed-By: Peter Eisentraut, Craig Ringer
Discussion: https://postgr.es/m/20161208213441.k3mbno4twhg2qf7g@alap3.anarazel.de

Author: anarazel