They're called "ceiling buttons", "plaster buttons", or "plaster washers". I found some at Home Depot, and saw listings for them when I plugged it into a search engine.
They're about the size of a nickle. They have little holes in them for the spackle to hang on to, and a big countersunk hole in the middle for a drywall screw.
If the ceiling is sagging, first gently prop it up to the level it should be. To install a ceiling button, drill a hole for the drywall screw a few inches away from the crack. Make sure you drill into lath so the screw has something to hold on to. Screw in the drywall screw and ceiling button. Spackle over it to conceal the repair. When your ceiling is secured, spackle the cracks and repair as usual.