Allow iteration and getting cache data for OVERLAY_TARGET_POINT

In this case, we're adding support for OVERLAY_TARGET_POINT. Logically,
this behaves similar to the more interesting case of
OVERLAY_TARGET_DYNAMIC. The main difference is that iter only ever
returns one entry, no matter where you start and get always returns the
same entry. libdladm will also tag these with a different kind of flag,
so dladm(1M) doens't have to learn too much about the implementation.