AVSTR — connect_leaf() operation

Summary: The connect_leaf() operation throws an exception if it is unable to use
ATM-style multipoint transport connections. Using the current scheme,
this happens, in principle, only when the first B party is bound into
the multiparty stream. It would be very useful if the StreamCtrl could first
test for the use of ATM-style multipoint by passing a nil object
reference for the ‘the_ep’ parameter in the call to connect_leaf().
It also leads to better semantics for binding the A party into the
multiparty stream.