Debug statements are not thread-safe

[davidben]

We probably should make them so, even though NPN_PluginThreadAsyncCall is the only thing that could be helped. In particular, the indent level wants to be thread-local storage and everything else should either also be thread-local or GOnce'd.

May be slightly more complicated if we don't want to assume the wrapper process has access to an initialized gthread. (Well, we could also assume pthreads.)