conditions) plab nodes without actually allocating them.

Changed init-elabnodes to support this.  Elab to plab mapping is set in the
NS file via the $opt array, which init-elabnodes extracts via the XMLRPC
interface.

So to run the "really simple model":

 1. set "no_plab" to 1 in the NS file
 2. make sure you have enough nodes listed in "plabnodes" in the NS file
    to cover "num_pcs".
 3. swapin the experiment
 4. after swapin, run init-elabnodes.pl.

Also, cleaned up the NS file some: got rid of hokey tcpdump setup as it
was redundant, made tracing optional (though on by default), expect proper
return values from program agents.