MINOR: conn_stream: new shutr/w status flags

MINOR: conn_stream: new shutr/w status flags

In order to support all shutdown modes on the CS, we introduce the
following flags :
  CS_FL_SHRD : shut read, drain extra data
  CS_FL_SHRR : shut read, reset extra data
  CS_FL_SHWN : shut write, normal notification
  CS_FL_SHWS : shut write, silent mode (no notification)

And the following modes for shutr/shutw :

  CS_SHR_DRAIN, CS_SHR_RESET, CS_SHW_NORMAL, CS_SHW_SILENT.

Note: it's possible that we won't need to distinguish the two shutw
above as they're only an action.

For now they are not used.