Display BGP gateway mode in show protocol output

Hello BIRD team! This simple patch (for branch master) adds a field for the BGP channel gateway mode (direct or recursive) setting in show protocol output. It is is a useful information to have in some troubleshooting scenario. Example: bird> show protocol all bgp_test Name Proto Table State Since Info bgp_test BGP --- up 2025-08-12 15:11:54 Established BGP state: Established [...] Channel ipv6 State: UP [...] Gateway: recursive [...] Thanks! -- Sébastien