MINOR: stream_interface: add reporting of ressouce allocation errors

SSL and keep-alive will need to be able to fail on allocation errors,
and the stream interface did not allow to report such a cause. The flag
will then be "RC" as already documented.
MINOR: session: report lack of resources using the new stream-interface's error code

Let's now use SI_ET_CONN_RES to report lack of resources instead of
SO_ET_CONN_OTHER with a handcrafted code.