plan9port OS X window labels

[Russ Cox <rsc@swtch.com>]

I fixed a bug in plan9port's OS X window
label handling the other day, so that 9term
windows will update their labels properly
in response to awd.

Unfortunately it seems that the label update
must be done inside the application event loop,
but it originates outside the event loop.
For now I have made the event loop check for
updates and apply them, so the label changes
at the next keystroke or mouse event.
The obvious solution is to generate a fake event
that is either a no-op of an existing message
or that the event loop can recognize and discard,
just so that the event loop notices the window
label has changed.  However, I could not get this
to work.  Perhaps there are Carbon experts here
who will have better luck.

http://hg.pdos.csail.mit.edu/hg/plan9/rev/12502ec69b95

Thanks.
Russ