implement operator* for cacheset iterators

implement operator* for cacheset iterators

Technically an abi-break as we change a template parameter to
std::iterator for this, but this class is empty in all instances and
just causes the right typedefs to be set – which were incorrect as
detected by std::stable_partition as its implementation uses ::pointer
and needs also a operator* implementation.

In practice CacheSets have no external users (yet) and the difference is
visible only at compile time (which was an error before and now works),
not while linking.

The changes to apt-mark are functionally identical to the code before,
just that we use a std:: algorithm now instead of trying hard on our
own.