* generic/udp_tcl.h: Done some testing with multicast and have

* generic/udp_tcl.h: Done some testing with multicast and have
* generic/udp_tcl.c: rationalised the fconfigure/udp_conf interfaces
a bit better. The -ttl option will set the multicast ttl if the
socket has been added to a multicast group.
Now maintains a list of groups so that this can be inspected and
when the socket is closed we will call mcastdrop for each item.