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 is not a plugin.

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

Reduce header dependencies.

For EXIT_SUCCESS.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.