This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Switch the remaining users to "postponed_exception".

Will replace "postponed_error".

Use the snd_pcm_t only in the IOThread, and reuse code that is
well-known to work.

New base class for other InputStream implementations that run in the
I/O thread.