implementing IN_PROGRESS

[Havoc Pennington]

Oh wait, in the shower just now I decided you're essentially right. The
behavior you proposed can be accomplished by just using a recursive lock
instead of a regular lock.

That will prevent two threads from dispatching at once, but allow
recursive dispatch in the same thread.

For some reason I was thinking this would be bad, but now that I spelled
everything out in long emails I don't think there's any reason it's
bad. 

The "bad reentrancy" would only happen if the app programmer or binding
called the main loop reentrantly on purpose, and even then a main loop
with a no recurse feature could prevent it. So a recursive dispatch lock
in DBusConnection will just serve to make things like
block_pending_call() work properly, and make recursive mainloops work
properly (when they are intentional)

We would need to do two things:
 - add recursive locks support to DBusThreadFunctions
   and update all bindings to provide them
 - audit the code that holds the dispatch lock to be 
   sure it's safe against same-thread reentrancy
   (e.g. messages being removed from the queue from 
   an application callback)

Havoc