Change the default value of max_prepared_transactions to zero, and add

documentation warnings against setting it nonzero unless active use of
prepared transactions is intended and a suitable transaction manager has been
installed.  This should help to prevent the type of scenario we've seen
several times now where a prepared transaction is forgotten and eventually
causes severe maintenance problems (or even anti-wraparound shutdown).

The only real reason we had the default be nonzero in the first place was to
support regression testing of the feature.  To still be able to do that,
tweak pg_regress to force a nonzero value during "make check".  Since we
cannot force a nonzero value in "make installcheck", add a variant regression
test "expected" file that shows the results that will be obtained when
max_prepared_transactions is zero.

Also, extend the HINT messages for transaction wraparound warnings to mention
the possibility that old prepared transactions are causing the problem.

All per today's discussion.

src/backend/utils/misc/postgresql.conf.sample

@@ -106,10 +106,12 @@
 #shared_buffers = 32MB			# min 128kB
 					# (change requires restart)
 #temp_buffers = 8MB			# min 800kB
-#max_prepared_transactions = 5		# can be 0 or more
+#max_prepared_transactions = 0		# zero disables the feature
 					# (change requires restart)
 # Note:  Increasing max_prepared_transactions costs ~600 bytes of shared memory
 # per transaction slot, plus lock space (see max_locks_per_transaction).
+# It is not advisable to set max_prepared_transactions nonzero unless you
+# actively intend to use prepared transactions.
 #work_mem = 1MB				# min 64kB
 #maintenance_work_mem = 16MB		# min 1MB
 #max_stack_depth = 2MB			# min 100kB