Code review for auto-tuned effective_cache_size.

Fix integer overflow issue noted by Magnus Hagander, as well as a bunch
of other infelicities in commit ee1e5662d8
and its unreasonably large number of followups.

src/backend/utils/misc/guc.c

@@ -4305,6 +4305,7 @@ SelectConfigFiles(const char *userDoption, const char *progname)
 	 */
 	pg_timezone_abbrev_initialize();
 
+	/* Also install the correct value for effective_cache_size */
 	set_default_effective_cache_size();
 
 	/*