refine encoder code for #536.

@@ -299,6 +299,10 @@ int SrsEncoder::initialize_ffmpeg(SrsFFMPEG* ffmpeg, SrsRequest* req, SrsConfDir
         log_file += req->app;
         log_file += "-";
         log_file += req->stream;
+        if (!engine->args.empty()) {
+            log_file += "-";
+            log_file += engine->arg0();
+        }
         log_file += ".log";
     }
 

@@ -102,6 +102,7 @@ int SrsFFMPEG::initialize_transcode(SrsConfDirective* engine)
 {
     int ret = ERROR_SUCCESS;
     
+    engine_name = engine->arg0();
     iformat             = _srs_config->get_engine_iformat(engine);
     vfilter             = _srs_config->get_engine_vfilter(engine);
     vcodec              = _srs_config->get_engine_vcodec(engine);
@@ -487,7 +488,7 @@ int SrsFFMPEG::start()
     // parent.
     if (pid > 0) {
         started = true;
-        srs_trace("vfored ffmpeg encoder engine, pid=%d", pid);
+        srs_trace("fork encoder %s, pid=%d", engine_name.c_str(), pid);
         return ret;
     }
     

@@ -49,6 +49,7 @@ private:
     bool fast_stopped;
     pid_t pid;
 private:
+    std::string engine_name;
     std::string log_file;
 private:
     std::string                 ffmpeg;