There are three pieces here, a change to the frisbee protocol itself, an
Emulab event component to get status back to the portal, and the surrounding
infrastructure to make it all work.

Frisbee heartbeat messages:

Added a new message type to the frisbee protocol, "Progress". In theory it
operates by having the server send a multicast progress request to its clients
which includes an interval at which to report (or "just once") and an
indication of what to report (nothing, progress summary, or full stats). The
client then sends unicast "fire and forget" UDP replies according to that
schedule. However, I took a shortcut for the moment and just added a command
line option to the client to tell it to report a summary at the indicated
interval (-H <interval>).  So the server never sends requests.

This is implemented in the client by a fourth thread since I wanted it to
operate independent of packet reception (which would cause clients to report
in a highly sync...