Merge ri_setnull and ri_setdefault into one function ri_set.  These
functions were to a large part identical.

This is a continuation in spirit of
4797f9b5.

Author: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/0ccdd3e1-10b0-dd05-d8a7-183507c11eb1%402ndquadrant.com