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".

ChangeLog

@@ -5,6 +5,9 @@ Changes in version 0.2.1.16-?? - 2009-??-??
     - Don't warn users about low port and hibernation mix when they
       provide a *ListenAddress directive to fix that. Bugfix on
       0.2.1.15-rc.
+    - Fix a race condition that could cause crashes or memory
+      corruption when running as a server with a controller listening
+      for log messages.
 
   o Minor bugfixes (on 0.2.1.x):
     - When switching back and forth between bridge mode, do not start