New implementation of EMsgPort

I would appreciate a review of my patch for bug #348888 [1].

The patch is a new implementation of EMsgPort using a GAsyncQueue.  It
also restores on-demand creation of pipes to keep file descriptor usage
from spiraling out of control at start-up, but without the race
conditions that forced the feature to be yanked earlier this year.  It
also keeps the existing API intact.

The new implementation seems pretty stable.  I've been running with it
for about a week now and haven't experienced any deadlocks or crashes.

Obviously we should wait until CVS is branched before committing it.

[1] http://bugzilla.gnome.org/show_bug.cgi?id=348888

Thanks,
Matthew Barnes