Get the AF_INET and friends constants from Socket

Apparently it is enough to simply import Socket or IO::Socket to use
AF_INET and friends, but all examples in official documentation show
them in the import list. Because I do not fully understand the
intricacies of Perl import logic, I do the same thing here, fully
aware that I might have joined a cargo cult. Regardless of its
correctness or necessity, listing the constants in the import list has
the advantage of making it clear why the `use` statement exists.

I chose to import the constants from Socket instead of IO::Socket
because that module's documentation explicitly documents the
constants.

configure.ac

@@ -40,7 +40,7 @@ m4_foreach_w([_m], [
     File::Basename
     File::Path
     Getopt::Long
-    IO::Socket
+    Socket
     Sys::Hostname
     version=0.77
   ], [AX_PROG_PERL_MODULES([_m], [],

ddclient.in

@@ -25,8 +25,8 @@ use warnings;
 use File::Basename;
 use File::Path qw(make_path);
 use Getopt::Long;
+use Socket qw(AF_INET AF_INET6 PF_INET PF_INET6);
 use Sys::Hostname;
-use IO::Socket;
 
 use version 0.77; our $VERSION = version->declare('v3.9.1');
 (my $version = $VERSION->stringify()) =~ s/^v//;