fix logrotate empty file hole issue

refer to ossrs#1554

trunk/etc/logrotate.d/srs

@@ -0,0 +1,15 @@
+/data/log/srs/*.log {
+
+weekly
+
+size 200M
+
+copytruncate
+
+rotate 5
+
+dateext
+
+noolddir
+
+}

trunk/src/app/srs_app_log.cpp

@@ -414,7 +414,7 @@ void SrsFastLog::open_log_file()
 
     if(fd == -1 && errno == ENOENT) {
         fd = open(filename.c_str(), 
-            O_RDWR | O_CREAT | O_TRUNC, 
+            O_RDWR | O_CREAT | O_TRUNC | O_APPEND/*logrotate copytruncate need this flag*/, 
             S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH
         );
     }