print_inet_addr: print nameless and named addresses using the same style

print_inet_addr: print nameless and named addresses using the same style

* sockaddr.c (print_inet_addr): Handle var_name == NULL case in a more
structured way: use inet_addr("%s") form to print AF_INET addresses
in structureless contexts where no structure field name is available.
Likewise, use inet_pton(AF_INET6, "%s") that case.
* tests/nlattr_ifaddrmsg.c (main): Update expected output.
* tests/nlattr_ifla_af_spec.c (main): Likewise.

Complements: v4.19~76 "print_inet_addr: add support of invocations without field name"