pgsql: Fix interaction between materializing holdable cursors and firing

Log Message:
-----------
Fix interaction between materializing holdable cursors and firing
deferred triggers: either one can create more work for the other,
so we have to loop till it's all gone. Per example from andrew(at)supernews(dot)
Add a regression test to help spot trouble in this area in future.

Modified Files:
--------------
pgsql/src/backend/access/transam:
xact.c (r1.198 -> r1.199)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.198&r2=1.199)
pgsql/src/backend/commands:
trigger.c (r1.183 -> r1.184)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.183&r2=1.184)
pgsql/src/backend/utils/mmgr:
portalmem.c (r1.77 -> r1.78)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mmgr/portalmem.c.diff?r1=1.77&r2=1.78)
pgsql/src/include/commands:
trigger.h (r1.52 -> r1.53)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h.diff?r1=1.52&r2=1.53)
pgsql/src/include/utils:
portal.h (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/portal.h.diff?r1=1.54&r2=1.55)
pgsql/src/test/regress/expected:
portals.out (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/portals.out.diff?r1=1.8&r2=1.9)
pgsql/src/test/regress/sql:
portals.sql (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/portals.sql.diff?r1=1.5&r2=1.6)