Add functions to count items on SM

In the interfaces where we have has_, get_, list_all_*

Add a count_* function, that will be needed for testing conformance, also useful to test locally. And it is a minor addition to the interfaces.

For instance on tests we need to check we have 0 opts, instead of calling list_all_opts() and counting items in the result, we can use count_opts() directly. This simplifies test implementation (e.g. in cucumber, spock or whatever tool).