ui-macos/main.py: fix wait() to avoid deadlock.

If the subprocess was trying to write to its stdout/stderr, its process
would never actually finish because it was blocked waiting for us to read
it, but we were blocked on waitpid().  Instead, use waitpid(WNOHANG) and
continually read from the subprocess (which should be a blocking operation)
until it exits.

src/ui-macos/main.py

@@ -97,7 +97,10 @@ class Runner:
         return self.rv
 
     def wait(self):
-        return self._try_wait(0)
+        rv = None
+        while rv is None:
+            self.gotdata(None)
+            rv = self._try_wait(os.WNOHANG)
 
     def poll(self):
         return self._try_wait(os.WNOHANG)