MINOR: conn-stream: Rely on endpoint shutdown flags to shutdown an applet

MINOR: conn-stream: Rely on endpoint shutdown flags to shutdown an applet

cs_applet_shut() now relies on CS_EP_SH* flags to performed the applet
shutdown. It means the applet release callback is called if there is no
CS_EP_SHR or CS_EP_SHW flags set. And it set these flags, CS_EP_SHRR and
CS_EP_SHWN more specifically, before exiting.

This way, cs_applet_shut() is the really equivalent to cs_conn_shut().