MINOR: receiver: add a receiver-specific flag to indicate the socket is bound

MINOR: receiver: add a receiver-specific flag to indicate the socket is bound

In order to split the receiver from the listener, we'll need to know that
a socket is already bound and ready to receive. We used to do that via
tha LI_O_ASSIGNED state but that's not sufficient anymore since the
receiver might not belong to a listener anymore. The new RX_F_BOUND flag
is used for this.