Due to recently introduces smart-pointers to Cap_index objects it's
necessary to always keep at least one reference as long as a corresponding
slot in the capability-space of a process is in use. This is especially
important for L4Linux that uses cap-slots directly without the given
abstractions of Genode.
This bug was introduced by commit c9c21ad39c, where Fiasco_capability
was removed, and enums defined in that class scope went to the namespace
Fiasco. In L4Linux some references to Fiasco_capability remained.
