Problem: report when unable to get a usable interface

On some setups, zbeacon cannot find a usable interface. The previous
behavior was to hide this problem from the caller. That caused tests
to hang mysteriously.

Now if the getnameinfo() call fails, the zbeacon constructor fails
and returns NULL. The caller cannot thus continue.
Fixed selftest for no-discovery case
Merge pull request #491 from hintjens/master

Problem: report when unable to get a usable interface