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/compat.h

@@ -522,6 +522,9 @@ void tor_threads_init(void);
 #define tor_threads_init() STMT_NIL
 #endif
 
+void set_main_thread(void);
+int in_main_thread(void);
+
 #ifdef TOR_IS_MULTITHREADED
 #if 0
 typedef struct tor_cond_t tor_cond_t;