* The current underlying DMA buffer Queue is not thread-safe and
in some cases it can corrupt the memory. Replace the underlying
DMA buffer container with a thread-safe implementation.

* Detect DAC underrun in available() and reset if needed. If DAC
stops for any reason, no buffers will become free again. We need
to detect this and abort.
* Flush all pending buffers when DAC is stopped. This releases back
all the buffers to the ready queue, and fixes an edge case where
DAC stops while holding all of the ready buffers.
* Check for NULL descriptor pointers in every function.
* Increase DMA stream priority to highest.

* Add new command to stop DAC ("k").
* Print memory usage.
* Increase max frequency to 128KHz.
* Use a smaller DMA buffer size and rework the wave generators
to support it.

Arguably, in user code, the list of pins to sample will typically be known and fixed at compile-time.

However, when wrapping this into a library (Mozzi; for the purpose of providing a cross-platform analog read mechanism), it will be very helpful to have a way to adjust the pins to sample after construction.

I shall test my PR before submitting...

Fixes and updates.

Allow setting list of pins to sample after construction