resolvectl: for our per-link verbs alternatively accept interface name in -i

resolvectl: for our per-link verbs alternatively accept interface name in -i

it's a bit confusing that we take two interfaces for verbs such as "dns"
or "domain": once after the verb, and once as --interface=. While
there's logic behind it, let's make this least surprising: if either is
specified be happy.

This means "resolvectl -i foo dns" is now equivalent to "resolvectl dns
foo …". Note that this is a tweak only, to minimize surprises. We don't
document this alternative syntax, and shouldn't to keep things simple.