Update tulip net driver to indicate link beat to system,

Update tulip net driver to indicate link beat to system,
via netif_carrier_{on,off}.  Some placeholders to do this were
already in the code, making this an even easier and more obvious patch.

Also, decrease time until next link beat check, if link beat
is not present.  (previously the code would wait 60 seconds until
next check, regardless of current link state)

Contributor: Stefan Rompf, with changes from me