Changes to open-file caching limits and uncached file unserialization.

- Unserializing files that were previously kicked out of the open-file
  cache would cause them to be fopen'd with the original access
  permissions which is usually 'w' and causes truncation.  They
  are now opened in 'a' mode.  (addresses #780)

- Add 'max_files_in_cache' script option to manually set the maximum
  amount of opened files to keep cached.  Mainly this just helped
  to create a simple test case for the above change.

- Remove unused NO_HAVE_SETRLIMIT preprocessor switch.

- On systems that don't enforce a limit on number of files opened for
  the process, raise default max size of open-file cache from
  32 to 512.

src/File.h

@@ -87,7 +87,13 @@ protected:
 
 	BroFile()	{ Init(); }
 	void Init();
-	bool Open(FILE* f = 0);	// if file is given, it's an open file to use
+
+	/**
+	 * If file is given, it's an open file to use already.
+	 * If file is not given and mode is, the filename will be opened with that
+	 * access mode.
+	 */
+	bool Open(FILE* f = 0, const char* mode = 0);
 
 	BroFile* Prev()	{ return prev; }
 	BroFile* Next()	{ return next; }