pgsql: Back-patch fixes for problems with VACUUM destroying t_ctid

Log Message:
-----------
Back-patch fixes for problems with VACUUM destroying t_ctid chains too soon,
and with insufficient paranoia in code that follows t_ctid links.
This patch covers the 7.4 branch.

Tags:
----
REL7_4_STABLE

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.157.2.1 -> r1.157.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.157.2.1&r2=1.157.2.2)
pgsql/src/backend/commands:
async.c (r1.102.2.1 -> r1.102.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/async.c.diff?r1=1.102.2.1&r2=1.102.2.2)
trigger.c (r1.159 -> r1.159.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.159&r2=1.159.2.1)
vacuum.c (r1.263.2.1 -> r1.263.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.263.2.1&r2=1.263.2.2)
pgsql/src/backend/executor:
execMain.c (r1.220.2.2 -> r1.220.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.220.2.2&r2=1.220.2.3)
pgsql/src/backend/utils/time:
tqual.c (r1.70.2.1 -> r1.70.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/time/tqual.c.diff?r1=1.70.2.1&r2=1.70.2.2)
pgsql/src/include/access:
heapam.h (r1.85.2.1 -> r1.85.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/heapam.h.diff?r1=1.85.2.1&r2=1.85.2.2)
pgsql/src/include/executor:
executor.h (r1.102.2.3 -> r1.102.2.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/executor.h.diff?r1=1.102.2.3&r2=1.102.2.4)