WireGuard's wq-quick configures such rules to avoid routing loops.
While we currently don't have an automatic solution for this, at least
we should support it via explicit user configuration.

One problem is that suppress_prefixlength is relatively new and kernel
might not support this attribute. That can lead to odd results, because
the NetworkManager is valid but it cannot be configured on the current
kernel. But this is a general problem, and we would require a general
solution. The solution cannot be to only support rule attributes that
are supported by the oldest possible kernel. It's not clear how much of
a problem there really is, or which general solution is required (if
any).