input/Stream: remove attribute "cond", replace with handler interface

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.

src/input/plugins/FileInputPlugin.hxx

@@ -24,10 +24,8 @@
 
 class Path;
 class Mutex;
-class Cond;
 
 InputStreamPtr
-OpenFileInputStream(Path path,
-		    Mutex &mutex, Cond &cond);
+OpenFileInputStream(Path path, Mutex &mutex);
 
 #endif