BUG/MINOR: proto_reverse_connect: set default maxconn

BUG/MINOR: proto_reverse_connect: set default maxconn

If maxconn is not set for preconnect, it assumes we want to establish a
single connection. However, this does not work properly in case the
connection is closed after reversal. Listener is not resumed by protocol
layer to attempt a new preconnect.

To fix this, explicitely set maxconn to 1 in the listener instance if
none is defined. This ensures the behavior is consistent. A BUG_ON() has
been added to validate we never try to use a listener with a 0 maxconn.