MINOR: stconn/connection: Move shut modes at the SE descriptor level

MINOR: stconn/connection: Move shut modes at the SE descriptor level

CO_SHR_* and CO_SHW_* modes are in fact used by the stream-connectors to
instruct the muxes how streams must be shut done. It is then the mux
responsibility to decide if it must be propagated to the connection layer or
not. And in this case, the modes above are only tested to pass a boolean
(clean or not).

So, it is not consistant to still use connection related modes for
information set at an upper layer and never used by the connection layer
itself.

These modes are thus moved at the sedesc level and merged into a single
enum. Idea is to add more modes, not necessarily mutually exclusive, to pass
more info to the muxes. For now, it is a one-for-one renaming.