MINOR: stconn: Use a dedicated function to get the opposite sedesc

MINOR: stconn: Use a dedicated function to get the opposite sedesc

se_opposite() function is added to let an endpoint retrieve the opposite
endpoint descriptor. Muxes supportng the zero-copy forwarding can now use
it. The se_shutdown() function too. This will be use by the SPOP multiplexer
to be able to retrieve the SPOE agent configuration attached to the applet
on client side.

The related issue is #2502.