Improving error handling for threads.

If a thread command fails (like the input framework not finding a
file), that now (1) no longer hangs Bro, and (2) even allows for
propagating error messages back before the thread is stops.

(Actually, the thread doesn't really "stop"; the thread manager keeps
threads around independent of their success; but it no longer polls
them for input.)

Closes #858.

src/threading/Manager.h

@@ -74,6 +74,16 @@ public:
 	 */
 	void ForceProcessing() { Process(); }
 
+	/**
+	 * Signals a specific threads to terminate immediately.
+	 */
+	void KillThread(BasicThread* thread);
+
+	/**
+	 * Signals all threads to terminate immediately.
+	 */
+	void KillThreads();
+
 protected:
 	friend class BasicThread;
 	friend class MsgThread;
@@ -106,13 +116,6 @@ protected:
 	 */
 	virtual double NextTimestamp(double* network_time);
 
-	/**
-	 * Kills all thread immediately. Note that this may cause race conditions
-	 * if a child thread currently holds a lock that might block somebody
-	 * else.
-	 */
-	virtual void KillThreads();
-
 	/**
 	 * Part of the IOSource interface.
 	 */