- 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.