Minor improvements to code for converting legacy CREATE CONSTRAINT TRIGGER

Minor improvements to code for converting legacy CREATE CONSTRAINT TRIGGER
commands into proper foreign-key constraints.  Believe the constraint name
given in the trigger arguments in preference to the trigger name --- to judge
from Olivier Prenant's example, pg_dump must at some time have used the
autogenerated trigger name there, though AFAICT no current release branch tip
does.  Improve the emitted NOTICEs to provide more detail (PK table's name and
column names).  Handle the case where pg_dump forgot to provide the FROM table
(a bug that never did get fixed in 7.0.x apparently).  This commit doesn't
do anything about the question of what to do with incomplete trigger groups.