[MEDIUM] session: make it possible to call an I/O handler on both SI

[MEDIUM] session: make it possible to call an I/O handler on both SI

This will be used when an I/O handler running in a stream interface
needs to establish a connection somewhere. We want the session
processor to evaluate both I/O handlers, depending on which side has
one. Doing so also requires that stream_int_update_embedded() wakes
the session up only when the other side is established or has closed,
for instance in order to handle connection errors without looping
indefinitely during the connection setup time.

The session processor still relies on BF_READ_ATTACHED being set,
though we must do whatever is required to remove this dependency.