Add 'use warnings' to tests that are missing it
Add an error constant for domain resolution failures

Instead of returning something like this on domain resolution failures:

    $ perl -MSocket=sockaddr_in,inet_aton -e 'sockaddr_in(80, inet_aton("EMPTY.lb-pool"))'
    Bad arg length for Socket::pack_sockaddr_in, length is 0, should be 4 at /home/v-perlbrew/perl5/perlbrew/perls/perl-5.19.6/lib/5.19.6/x86_64-linux/Socket.pm line 833

We'll now return a new error constant indicating that we couldn't
resolve the domain.

Also change sockaddr_in to pack_sockaddr_in, the former is just a
relatively pointless wrapper around the latter when called in scalar
context.

Ad-hoc command-line I used to validate that:

    $ perl -MData::Dumper -MSocket=pack_sockaddr_in,sockaddr_in,inet_ntoa,inet_aton -wle 'my $gethostbyname= gethostbyname("google.com"); my $inet_aton = inet_aton("google.com"); my $inet_ntoa = inet_ntoa($gethostbyname); my $sockaddr_in = sockaddr_in(80, $inet_aton); my $pack_sockaddr_in = pack_sockaddr_in(80, $inet_aton); print Dumper { gethostbyname => $gethostbyname, inet_aton => $inet_aton, inet_ntoa => $inet_ntoa, sockaddr_in => $sockaddr_in, pack_sockaddr_in => $pack_sockaddr_in }'
    $VAR1 = {
              'sockaddr_in' => 'P��Ff',
              'gethostbyname' => '��Ff',
              'pack_sockaddr_in' => 'P��Ff',
              'inet_aton' => '��Ff',
              'inet_ntoa' => '173.194.70.102'
            };
Bump the version to 0.12