Support for cleaning up threads that have terminated.

Once a BasicThread leaves its run() method, a thread is now marked for
cleaning up, and the ThreadMgr will soon join it to release the OS
resources.

Also, adding a function Log::remove_stream() that remove a logging
stream, stopping all writer threads that are associated with it.

Note, however, that removing a *filter* from a stream still doesn't
clean up any threads. The problem is that because of the output paths
potentially being created dynamically it's unclear if the writer
thread will still be needed in the future. We could add clean writers
up with timeouts, but that doesn't sound great either. So for now, the
only way to sure clean up logging threads is to remove the entire
stream.

Also note that cleanup doesn't work with input threads yet, which
don't seem to terminate (at least in the case I tried).

src/logging/Manager.h

@@ -47,6 +47,16 @@ public:
 	 */
 	bool CreateStream(EnumVal* id, RecordVal* stream);
 
+	/**
+	 * Remove a log stream, stopping all threads.
+	 *
+	 * @param id  The enum value corresponding the log stream.
+	 *
+	 * This methods corresponds directly to the internal BiF defined in
+	 * logging.bif, which just forwards here.
+	 */
+	bool RemoveStream(EnumVal* id);
+
 	/**
 	 * Enables a log log stream.
 	 *