The internal set call is volatile by itself because it requires reading
the value that may have changed. But the wrapper class is just itself.

Added doxygen for find_suitable_fragment.

The iterator in the map is now completely wrapped into another class, to
make its usage from outside more sensible. The old fragment object has
become a simple payload struct (frag_data) and the obscure pfrag class
is now simply called frag_iterator. The function to test whether a pfrag
is valid could now be removed.