Make old posts visible only for the circle on Google+

The settings are still there when you post.

In the case below I have a list called Locked and I allow everyone except those in that list.

You can always edit this by clicking on it.

From there the same way that you will reference a friend, is the same way you would reference a list. Just type the list name. Below I used it in the hide list, but you could do the same with "Make this visible to" , just select "Specific People"

So you could say the "These people" could have been a bit more descriptive and said "These people and lists"

For older posts there is a little lock icon that shows the privacy settings for that particular item.

This has been fixed recently. Go to your Circles, right click on the circle you want to throttle or mute, and select "Show stream from this circle". At the top of the stream you will notice a slider that lets you adjust how much you want to see from that stream, including all and nothing.