Copter: set UARTs non-blocking at end of initialisation

Previously we were setting to non-block after arming but this reduce the
chance of bumping into a NuttX USB driver issue that can cause the uart
to become unresponsive and also makes Copter consistent with Plane
AC_AttControl: re-enable rate feedforward by default

Roll and Pitch rate controllers became sluggish without feed-forward
enabled.