repmgr: parse --no-slot in pg_basebackup_options

From PostgreSQL 10 we'll need to know whether this is present when
performing sanity checks for available replication slots.

Add a sanity check for conflicting presence of -S/--slot while we're
at it so we can abort early.

repmgr.h

@@ -193,9 +193,10 @@ typedef struct
 {
 	char		slot[MAXLEN];
 	char		xlog_method[MAXLEN];
+	bool		no_slot; /* from PostgreSQL 10 */
 } t_basebackup_options;
 
-#define T_BASEBACKUP_OPTIONS_INITIALIZER { "", "" }
+#define T_BASEBACKUP_OPTIONS_INITIALIZER { "", "", false }
 
 typedef struct
 {