Add usnic connectivity-checking agent service.
    
Basically: since usnic is a connectionless transport, we do not get
OS-provided services "for free" that connection-oriented transports
get, namely: "hey, I wasn't able to make a connection to peer X", and
"hey, your connection to peer X has died."
    
This connectivity-checker runs in a separate progress thread in the
usnic BTL in local rank 0 on each server.  Upon first send in any
process, the connectivty-checker agent will send some UDP pings to the
peer to ensure that we can reach it.  If we can't, we'll abort the job
with a nice show_help message.
    
There's a lengthy comment in btl_usnic_connectivity.h explains the
scheme and how it works.

Reviewed by Dave Goodell.

cmr=v1.7.5:ticket=#4253