[Win32] Define some Socket-related %Config keys

[sisyphus]

Affects only mingw-w64 builds of perl.
With mingw-w64, the Socket module that is built during the building of perl has lacked some features that are provided by a Socket module that is built and installed subsequent to the building of perl.
This PR now gives us the fully featured Socket module when built as part of the perl build.

Fixes StrawberryPerl/Perl-Dist-Strawberry#221
Also closes https://rt.cpan.org/Public/Bug/Display.html?id=149072

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• This set of changes does not require a perldelta entry.

[sisyphus]

For your convenience, a little script based on the scripts provided by @Yaribz when reporting the issues to Strawberry Perl:

use strict;
use warnings;

use Socket qw'getaddrinfo AF_INET6';

my ($err)=getaddrinfo('perl.org', 80, {socktype => -1});
if($err) {print "1: getaddrinfo 'OK'\n" }
else {print "1: getaddrinfo 'NOT ok'\n" }

($err)=getaddrinfo('www.google.com', 443, {family => AF_INET6});
if($err) {print "2: getaddrinfo 'NOT ok'\n"}
else {print "2: getaddrinfo 'OK'\n"}

If perl is built with the patches provided here, that script outputs:

1: getaddrinfo 'OK'
2: getaddrinfo 'OK'

Else it outputs:

1: getaddrinfo 'NOT ok'
2: getaddrinfo 'NOT ok'

Admittedly, I suspect that little script does not demonstrate the full range of benefits that the patch provides.