ip netconf: Show all address families by default in dumps

ip netconf: Show all address families by default in dumps

Currently, 'ip netconf' only shows ipv4 and ipv6 netconf settings. If IPv6
is not enabled, the dump ends with
    RTNETLINK answers: Operation not supported

when IPv6 request is attempted. Further, if the mpls_router module is also
loaded a separate request is needed to get MPLS settings.

To make this better going forward, use the new PF_UNSPEC dump all option
if the kernel supports it. If the kernel does not, it sets NLMSG_ERROR and
returns EOPNOTSUPP which is trapped and we fall back to the existing output
to maintain compatibility with existing kernels.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>