Several updates to libplab.py and plabnode.in
- getfree daemon doesn't die anymore when communcation with the plab dslice agent fails. - the link classifier logic has been changed slightly to allow nodes to be classified as inet2 even if they don't reverse resolve. The problem here is that intl nodes that don't resolve, but which go through abilene will look like inet2 nodes, which is wrong. Manual verification of the node_auxtypes table is still recommended. - The fping verifier has been disabled for now (since some plab nodes block ICMP traffic). - made some error messages more descriptive - plabnodes script now handles more agent communication errors gracefully (retries when if encounters them). - rearranged plabnode's retry loops to be a little easier to read, and more general.