Add CLI/API feature for modifying link state.

[rcgoodfellow]

The capability to modify simnet link-state was added to illumos here

• https://www.illumos.org/issues/14122

This should be added as a first-class feature to the Falcon CLI and API. This is a little bit tricky as multiple links can exist between the same two nodes. So something like

<topology> link down node1 node2

would not be sufficient. It'd need to be something a bit more verbose like

<topology> link down node1 node2 1

to indicate the index of the link between the nodes, or we could introduce something like link tags and then do something like

<topology> link down node1 node2 primary