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.
Make the "open" method of plugins "file" and "archive" dummy methods
that always fail. Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream(). This allows
simplifyin the algorithm for falling back to probing archive plugins.
