(maint) Fix crash when missing interface name or address.

When enumerating interfaces in the BSD network facts provider, check
that ifa_addr and ifa_name are populated for the interface. These
members are allowed to be nullptr.

Fixing the unnecessary copy of the netmask for IPv6 address-to-string
conversions.
Replacing the use of inet_ntoa with inet_ntop.
(maint) Fix inability to lookup netmask6 and network6 facts.

The networking facts provider did not include the netmask6[_*] and
network6[_*] facts in its constructor.  As a result, we couldn't query
these facts directly on cfacter's command line.
(maint) Enable better crash reporting in Travis.

For the native unit tests, use libSegFault.so to print stack traces for
a crash.

For both the native and ruby tests, pipe the output through c++filt,
which will demangle C++ symbols in any stack traces.