_return() assigns the return value @retval and asserts that
currently no return-value is assigned -- in order not to overwrite
a once set value.

That requires, that we call _return() and exit the main-loop right
away, without possibility to call _return() again.

However, during client_properties_changed() we easily might
receive the "notify" signal multiple times, and thus call
quit_if_connected() and _return() multiple times. That would
lead to a wrong assertion failure.

Avoid that, by disconnecting the signal handler once we reach
_return().

Fixes: f7875a42