Support multiple RADIUS servers

This changes all the RADIUS related parameters (radiusserver,
radiussecret, radiusport, radiusidentifier) to be plural and to accept a
comma separated list of servers, which will be tried in order.

Reviewed by Adam Brightwell

src/include/libpq/hba.h

@@ -89,10 +89,14 @@ typedef struct HbaLine
 	bool		include_realm;
 	bool		compat_realm;
 	bool		upn_username;
-	char	   *radiusserver;
-	char	   *radiussecret;
-	char	   *radiusidentifier;
-	int			radiusport;
+	List	   *radiusservers;
+	char       *radiusservers_s;
+	List	   *radiussecrets;
+	char       *radiussecrets_s;
+	List	   *radiusidentifiers;
+	char       *radiusidentifiers_s;
+	List	   *radiusports;
+	char       *radiusports_s;
 } HbaLine;
 
 typedef struct IdentLine