Lower *_freeze_max_age minimum values.

The old minimum values are rather large, making it time consuming to
test related behaviour. Additionally the current limits, especially for
multixacts, can be problematic in space-constrained systems. 10000000
multixacts can contain a lot of members.

Since there's no good reason for the current limits, lower them a good
bit. Setting them to 0 would be a bad idea, triggering endless vacuums,
so still retain a limit.

While at it fix autovacuum_multixact_freeze_max_age to refer to
multixact.c instead of varsup.c.

Reviewed-By: Robert Haas
Discussion: CA+TgmoYmQPHcrc3GSs7vwvrbTkbcGD9Gik=OztbDGGrovkkEzQ@mail.gmail.com
Backpatch: 9.0 (in parts)

src/backend/utils/misc/guc.c

@@ -2264,7 +2264,7 @@ static struct config_int ConfigureNamesInt[] =
 		},
 		&autovacuum_freeze_max_age,
 		/* see pg_resetxlog if you change the upper-limit value */
-		200000000, 100000000, 2000000000,
+		200000000, 100000, 2000000000,
 		NULL, NULL, NULL
 	},
 	{