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.

Allow implementations to install the "real" input later.

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

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