Threaded logging framework.

This is based on Gilbert's code but I ended up refactoring it quite a
bit. That's why I didn't do a direct merge but started with a new
branch and copied things over to adapt. It looks quite a bit different
now as I tried to generalize things a bit more to also support the
Input Framework.

The larger changes code are:

    - Moved all logging code into subdirectory src/logging/. Code
      here is in namespace "logging".

    - Moved all threading code into subdirectory src/threading/. Code
      here is in namespace "threading".

    - Introduced a central thread manager that tracks threads and is
      in charge of termination and (eventually) statistics.

    - Refactored logging independent threading code into base classes
      BasicThread and MsgThread. The former encapsulates all the
      pthread code with simple start/stop methods and provides a
      single Run() method to override.

      The latter is derived from BasicThread and adds bi-directional
      message passing between main and child threads. The hope is that
      the Input Framework can reuse this part quite directly.

    - A log writer is now split into a general WriterFrontend
      (LogEmissary in Gilbert's code) and a type-specific
      WriterBackend. Specific writers are implemented by deriving from
      the latter. (The plugin interface is almost unchanged compared
      to the 2.0 version.).

      Frontend and backend communicate via MsgThread's message
      passing.

    - MsgThread (and thus WriterBackend) has a Heartbeat() method that
      a thread can override to execute code on a regular basis. It's
      triggered roughly once a second by the main thread.

    - Integration into "the rest of Bro". Threads can send messages to
      the reporter and do debugging output; they are hooked into the
      I/O loop for sending messages back; and there's a new debugging
      stream "threading" that logs, well, threading activity.

This all seems to work for the most part, but it's not done yet.

TODO list:

    - Not all tests pass yet. In particular, diffs for the external
      tests seem to indicate some memory problem (no crashes, just an
      occasional weird character).

    - Only tested in --enable-debug mode.

    - Only tested on Linux.

    - Needs leak check.

    - Each log write is currently a single inter-thread message. Bring
      Gilbert's bulk writes back.

    - Code needs further cleanup.

    - Document the class API.

    - Document the internal structure of the logging framework.

    - Check for robustness: live traffic, aborting, signals, etc.

    - Add thread statistics to profile.log (most of the code is there).

    - Customize the OS-visible thread names on platforms that support it.

src/threading/Manager.h

@@ -0,0 +1,52 @@
+
+#ifndef THREADING_MANAGER_H
+#define THREADING_MANAGER_H
+
+#include <list>
+
+#include "IOSource.h"
+
+#include "BasicThread.h"
+#include "MsgThread.h"
+
+namespace threading {
+
+class Manager : public IOSource
+{
+public:
+	Manager();
+	~Manager();
+
+	void Terminate();
+
+protected:
+	friend class BasicThread;
+	friend class MsgThread;
+
+	void AddThread(BasicThread* thread);
+	void AddMsgThread(MsgThread* thread);
+
+	// IOSource interface.
+	virtual void GetFds(int* read, int* write, int* except);
+	virtual double NextTimestamp(double* network_time);
+	virtual void Process();
+	virtual const char* Tag()	{ return "threading::Manager"; }
+
+private:
+	static const int HEART_BEAT_INTERVAL = 1;
+
+	typedef std::list<BasicThread*> all_thread_list;
+	all_thread_list all_threads;
+
+	typedef std::list<MsgThread*> msg_thread_list;
+	msg_thread_list msg_threads;
+
+	bool did_process;
+	double next_beat;
+};
+
+}
+
+extern threading::Manager* thread_mgr;
+
+#endif