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".