Handle ProcessChain in open(f, cmd, ..)

[hexaeder]

Hey all,

On current master, open(f, cmds::pipeline) does not forward an exception inf but throws a MethodError since waitkill is only defined for Process. This PR extends it to ProcessChain. Maybe the type specification could be dropped entirely...

cmd = pipeline(`sleep 100`, `echo "foo"`)
open(cmd) do out
    error("my error")
end
# throws MethodError

cmd = `sleep 100`
open(cmd) do out
    error("my error")
end
# throws "my error" as expected

[vtjnash]

(regression introduced by #44078)