IPRoute.flush_addr(altname='some-alt-name') does not work

[socketpair]

See #1065

neither IPRoute.flush_addr(altname='some-alt-name') nor IPRoute.flush_addr(alt_ifname='some-alt-name') does not work (just ignores this kwarg)

Moreover, IPRoute.flush_addr(random_kwarg='anything') does not fail (that's another story).