Add an option to set source address for a BGP session.

When adding a BGP neighbour, one can set "update-source" (quagga syntax) to the
source IP address that will be used for that session.

Signed-off-by: Benjamin Cama <benoar@dolka.fr>

l2tpns.h

@@ -723,6 +723,7 @@ typedef struct
 	    	uint16_t as;
 		int keepalive;
 		int hold;
+		struct in_addr update_source;
 	} neighbour[BGP_NUM_PEERS];
 #endif
 } configt;