When constructing a thread object its capability is inserted into the
capability map. Normally this is done by the ipc-unmarshalling code, but
in this case the thread-capability isn't transfered via normal IPC, but in
a special form via the thread_state object. In contrast to the unmarshalling
code, the thread-startup code doesn't check, whether the capability-map
already contains a deprecated entry with the same capability id before
inserting the thread's capability. This commit add the necessary check.
Moreover, a check is added to the insertion methods of the capability-map
to verify that capability-allocation didn't failed.
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