REORG: connection: rename CO_FL_DATA_* -> CO_FL_XPRT_*

REORG: connection: rename CO_FL_DATA_* -> CO_FL_XPRT_*

These flags are not exactly for the data layer, they instead indicate
what is expected from the transport layer. Since we're going to split
the connection between the transport and the data layers to insert a
mux layer, it's important to have a clear idea of what each layer does.

All function conn_data_* used to manipulate these flags were renamed to
conn_xprt_*.