Fixes those programs on Windows.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

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.

Eliminate dependency on io_thread_get().