Update variable/field names relating to pg_basebackup's -X option

Now the "xlog nomenclature" Pg versions are fading into the past,
rename things related to handling pg_basebackup's -X option
(was: --xlog-method, now: --wal-method) to start with "wal_"
rather than "xlog_".

This is a cosmetic change for code clarity.

configfile.h

@@ -251,7 +251,7 @@ typedef struct
 typedef struct
 {
 	char		slot[MAXLEN];
-	char		xlog_method[MAXLEN];
+	char		wal_method[MAXLEN];
 	bool		no_slot;		/* from PostgreSQL 10 */
 } t_basebackup_options;