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/BasicThread.h

@@ -0,0 +1,63 @@
+
+#ifndef THREADING_BASICTHREAD_H
+#define THREADING_BASICTHREAD_H
+
+#include <pthread.h>
+#include <semaphore.h>
+
+#include "Queue.h"
+#include "util.h"
+
+using namespace std;
+
+namespace threading {
+
+class Manager;
+
+class BasicThread
+{
+public:
+	BasicThread(const string& name); // Managed by manager, must not delete otherwise.
+	virtual ~BasicThread();
+
+	const string& Name() const { return name; }
+
+	void Start(); // Spawns the thread and enters Run().
+	void Stop();  // Signals the thread to terminate.
+
+	bool Terminating()  const { return terminating; }
+
+	// A thread-safe version of fmt().
+	const char* Fmt(const char* format, ...);
+
+protected:
+	virtual void Run() = 0;
+
+	virtual void OnStart()	{}
+	virtual void OnStop()	{}
+
+private:
+	friend class Manager;
+
+	static void* launcher(void *arg);
+
+	// Used from the ThreadMgr.
+	void Join();	// Waits until the thread has terminated and then joins it.
+
+	bool started; 		// Set to to true once running.
+	bool terminating;	// Set to to true to signal termination.
+	string name;
+
+	pthread_t pthread;
+	sem_t terminate;
+
+	// For implementing Fmt().
+	char* buf;
+	unsigned int buf_len;
+};
+
+}
+
+extern threading::Manager* thread_mgr;
+
+#endif