mcast addr problem, change the pick_address code so that it always
adds one to the last address picked. Store the last used index in the
emulab_indicies table (the table is seeded with the BASEADDR if there
is no existing address, so external emulabs will be okay). With the
lower three octets, it should be quite some time before we run out and
have to start over (currently it sends email if this happens).