network: Support interface-bound ECMP routes in MultiPathRoute= (#39742)

MultiPathRoute= can now specify device-only nexthops without a gateway
address, e.g. MultiPathRoute=@wg0. This enables ECMP configurations over
interfaces that don't use gateway addresses, such as WireGuard tunnels.

The syntax is extended from "address[@device] [weight]" to
"[address]@device [weight]". The address is now optional, but at least
one of gateway or device must be specified. The @ symbol must still be
present for device-only routes, making the syntax unambiguous: @wg0
specifies a device, while a bare IP address specifies a gateway.

Device-only nexthops are only available for IPv4 routes. Device-only
multipath routes for IPv6 are not supported by the kernel's netlink
interface and will be rejected with a warning.

This change is fully backwards compatible. All existing configurations
continue to work unchanged, as they always included a gateway address.

Closes #39699.

test/test-network/conf/25-route-static.network

@@ -115,3 +115,8 @@ Peer=1.1.8.104/31
 
 [Route]
 Gateway=1.1.8.104
+
+[Route]
+Destination=192.168.20.0/24
+MultiPathRoute=@test1 15
+MultiPathRoute=@dummy98 25

test/test-network/systemd-networkd-tests.py

@@ -4392,6 +4392,13 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         self.assertIn('via 2001:1234:5:8fff:ff:ff:ff:ff dev dummy98', output)
         self.assertIn('via 2001:1234:5:9fff:ff:ff:ff:ff dev dummy98', output)
 
+        print('### ip route show 192.168.20.0/24')
+        output = check_output('ip route show 192.168.20.0/24')
+        print(output)
+        self.assertIn('192.168.20.0/24 proto static', output)
+        self.assertIn('nexthop dev test1 weight 15', output)
+        self.assertIn('nexthop dev dummy98 weight 25', output)
+
         check_json(networkctl_json())
 
     def _check_unreachable_routes_removed(self):