#297 Proxy does not remove handles from map.

[Sean Hefty]

If UM objects are destroyed out of order (e.g. CA before
PD), the proxy will keep the underlying handles in its
map and validate destroyed kernel objects. Proxy
handles should be removed from the map when the
objects are destroyed by pfn_destroy, rather than just
returning from API calls. That is, make
proxy_put_handle part of the destroying operation of AL
objects, rather than done specifically by the proxy.