CLEANUP: stconn: rename cs_rx_room_{blk,rdy} to sc_{need,have}_room()

CLEANUP: stconn: rename cs_rx_room_{blk,rdy} to sc_{need,have}_room()

The new name mor eclearly indicates that a stream connector cannot make
any more progress because it needs room in the channel buffer, or that
it may be unblocked because the buffer now has more room available. The
testing function is sc_waiting_room(). This is mostly used by applets.
Note that the flags will change soon.