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.

diff --cc src/network/networkd-route-nexthop.c
Simple merge

diff --cc src/network/test-networkd-conf.c
Simple merge

diff --cc test/test-network/systemd-networkd-tests.py
Simple merge