And specify whether generic tags are being scanned.

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.

Type-safe wrapper for uri_get_suffix().

Replaces the method Close().

Merge some duplicate code.

Got lost in commit c97685fe

.. instead of decoder_plugin_from_suffix().  This reduces overhead by
walking the array only once.

Allows forward-declaration.