pgsql: Use schema search path to find the first matching contraint name

Log Message:
-----------
Use schema search path to find the first matching contraint name for SET
CONSTRAINT, rather than affecting all constraints in all schemas (which
is what we used to do). Also allow schema specifications.

Kris Jurka

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
set_constraints.sgml (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/set_constraints.sgml.diff?r1=1.12&r2=1.13)
pgsql/src/backend/commands:
trigger.c (r1.200 -> r1.201)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.200&r2=1.201)
pgsql/src/backend/parser:
gram.y (r2.542 -> r2.543)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.542&r2=2.543)
pgsql/src/include/nodes:
parsenodes.h (r1.307 -> r1.308)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h.diff?r1=1.307&r2=1.308)