The sctp scoping rules try to prevent accidentally sending
non-routable addresses to a peer as alternate addresses.  However,
the current implementation is way too strict in that one can't
even try to send from a private address to a global address (a
pretty common real world setup with host behind NAT) even 
when that's the _only_ address available.

If we find no addresses available, try looking for a private address 
to send with, as a fallback.  
Additionally, if we are sending with only a single address, don't
bother embedding this address in the INIT/INIT-ACK chunk.