Don't attempt to log messages to a controller from a worker thread.

This patch adds a function to determine whether we're in the main
thread, and changes control_event_logmsg() to return immediately if
we're in a subthread.  This is necessary because otherwise we will
call connection_write_to_buf, which modifies non-locked data
structures.

Bugfix on 0.2.0.x; fix for at least one of the things currently
called "bug 977".

src/common/util.c

@@ -2480,6 +2480,8 @@ start_daemon(void)
     if (fork() != 0) {
       exit(0);
     }
+    set_main_thread(); /* We are now the main thread. */
+
     return;
   }
 }