A port conn is now upgraded to an yb conn only if the port can't be used.

This fixes a problem where selecting a port connection, whose programmer
type is set to shupito, into a terminal fails.
PortShupitoConn will now request the descriptor on second connect too.
USB ACM line params will be set before the conn is marked connected.

This makes any yb conns over the ACM port request descriptors only after
the line is prepared.
Disabled double-buffering in ACM conns.

The bug in the driver seems to be triggered even with only two buffers.