For WireGuard (like for all IP-tunnels and IP-based VPNs), the IP addresses of
the peers must be reached outside the tunnel/VPN itself.

For VPN connections, NetworkManager usually adds a direct /32 route to
the external VPN gateway to the underlying device. For WireGuard that is
not done, because injecting a route to another device is ugly and error
prone. Worse: WireGuard with automatic roaming and multiple peers makes this
more complicated.

This is commonly a problem when setting the default-route via the VPN,
but there are also other subtle setups where special care must be taken
to prevent such routing loops.

WireGuard's wg-quick provides a simple, automatic solution by adding two policy
routing rules and relying on the WireGuard packets having a fwmark set (see [1]).

Let's also do that. Add new properties "wireguard.ip4-auto-default-route"
and "wireguard.ip6-auto-default-route" to enable/disable this. Note that
the default value...