Add tests for actually receiving multiple signals and multiple polls at once, not just listening for them.

fix typos in error message and comment

Add zero arg constructor and `getType` to DynamicValue::Pipeline

Make Schema default constructors produce consistent, non-crashy values.  This improves the dynamic interface's robustness, particularly when exceptions are disabled.

Refactor capability code using fork.  Still too much refcounting, though.  Maybe this calls for a different design for pipelining...