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 8.0 branch.

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.182 -> r1.182.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.182&r2=1.182.4.1)
pgsql/src/backend/commands:
async.c (r1.118 -> r1.118.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/async.c.diff?r1=1.118&r2=1.118.4.1)
trigger.c (r1.177.4.1 -> r1.177.4.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.177.4.1&r2=1.177.4.2)
vacuum.c (r1.299 -> r1.299.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.299&r2=1.299.4.1)
pgsql/src/backend/executor:
execMain.c (r1.241 -> r1.241.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.241&r2=1.241.4.1)
pgsql/src/backend/utils/time:
tqual.c (r1.81.4.1 -> r1.81.4.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/time/tqual.c.diff?r1=1.81.4.1&r2=1.81.4.2)
pgsql/src/include/access:
heapam.h (r1.93 -> r1.93.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/heapam.h.diff?r1=1.93&r2=1.93.4.1)
pgsql/src/include/executor:
executor.h (r1.115 -> r1.115.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/executor.h.diff?r1=1.115&r2=1.115.4.1)