IOSource reorg.

A bunch of infrastructure work to move IOSource, IOSourceRegistry (now
iosource::Manager) and PktSrc/PktDumper code into iosource/, and over
to a plugin structure.

Other IOSources aren't touched yet, they are still in src/*.

It compiles and does something with a small trace, but that's all I've
tested so far. There are quite certainly a number of problems left, as
well as various TODOs and cleanup; and nothing's cast in stone yet.

Will continue to work on this.

src/DebugLogger.h

@@ -29,6 +29,7 @@ enum DebugStream {
 	DBG_FILE_ANALYSIS,	// File analysis
 	DBG_PLUGINS,
 	DBG_BROXYGEN,
+	DBG_PKTIO,	// Packet sources and dumpers.
 
 	NUM_DBGS // Has to be last
 };