The following fixes partly solve the problems triggered by the noux stress
test introduced by nfeske in issue #208.
* The check whether a capability exists in the Cap_map, and its insertion,
has to be done atomically
* While removing a capability it is looked up in the Cap_map via its id,
check whether the found capability pointer is the same like the looked up,
otherwise the wrong capability gets freed
* When a local capability is un- resp. marshalled, only the local pointer
gets transfered, not the redundant capability id
* Introduce several assertions and warnings to facilitate debugging
