Again fix initialization of auto-tuned effective_cache_size.

The previous method was overly complex and underly correct; in particular,
by assigning the default value with PGC_S_OVERRIDE, it prevented later
attempts to change the setting in postgresql.conf, as noted by Jeff Janes.
We should just assign the default value with source PGC_S_DYNAMIC_DEFAULT,
which will have the desired priority relative to the boot_val as well as
user-set values.

There is still a gap in this method: if there's an explicit assignment of
effective_cache_size = -1 in the postgresql.conf file, and that assignment
appears before shared_buffers is assigned, the code will substitute 4 times
the bootstrap default for shared_buffers, and that value will then persist
(since it will have source PGC_S_FILE).  I don't see any very nice way
to avoid that though, and it's not a case to be expected in practice.
The existing comments in guc-file.l look forward to a redesign of the
DYNAMIC_DEFAULT mechanism; if that ever happens, we should consider this
case as one of the things we'd like to improve.

src/backend/utils/misc/guc-file.l

@@ -298,6 +298,7 @@ ProcessConfigFile(GucContext context)
 	{
 		InitializeGUCOptionsFromEnvironment();
 		pg_timezone_abbrev_initialize();
+		set_default_effective_cache_size();
 		/* this selects SQL_ASCII in processes not connected to a database */
 		SetConfigOption("client_encoding", GetDatabaseEncodingName(),
 						PGC_BACKEND, PGC_S_DYNAMIC_DEFAULT);