matchBinaries follow-ups

[tpapagian]

From 0854a3b:

As a follow-up we can also clear entries from the (shared) names_map
when we remove kprobes. For now we also increase the size of that map to
256 entries. This means that we can define up to 256 unique binary names
among all matchBinaries selectors.

Since the same name may be used by multiple kprobes, we would need reference counting to know when to remove an entry. Or we can just scan everything.

Another limitation is that we only catch binaries that are exec-ed after the policy started.

More details in #686 (review)