pgsql: Change TRUNCATE's method for searching for foreign-key references

Log Message:
-----------
Change TRUNCATE's method for searching for foreign-key references so that
the order in which it visits tables is not dependent on the physical order
of pg_constraint entries, and neither are the error messages it gives.
This should correct recently-noticed instability in regression tests.

Modified Files:
--------------
pgsql/src/backend/catalog:
heap.c (r1.301 -> r1.302)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c.diff?r1=1.301&r2=1.302)
pgsql/src/test/regress/expected:
temp.out (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/temp.out.diff?r1=1.11&r2=1.12)
truncate.out (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/truncate.out.diff?r1=1.11&r2=1.12)
pgsql/src/test/regress/sql:
truncate.sql (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/truncate.sql.diff?r1=1.4&r2=1.5)