Core: `ignore_unavailable` shouldn't ignore closed indices

The `ignore_unavailable` request setting shouldn't ignore closed indices if a single index is specified in a search or broadcast request.

PR for #7153
I didn't do this, because otherwise I had to copy-paste this in this method: 
```java
boolean failClosed = indicesOptions.forbidClosedIndices() && !indicesOptions.ignoreUnavailable();
```

But i'm ok with adding it.