Rename "logxxx" configuration file parameters to "log_xxx"

This is more consistent with other parameters and conforms to
the pattern used by PostgreSQL itself, which uses the prefix "log_"
for logging parameters.

A warning will be emitted if the old version of the parameter name
is detected.

config.h

@@ -59,9 +59,9 @@ typedef struct
 	int			replication_type;
 
 	/* log settings */
-	char		loglevel[MAXLEN];
-	char		logfacility[MAXLEN];
-	char		logfile[MAXLEN];
+	char		log_level[MAXLEN];
+	char		log_facility[MAXLEN];
+	char		log_file[MAXLEN];
 
 	/* standby clone settings */
 	bool		use_replication_slots;