Add configure check for <sys/sockio.h>

Some systems need <sys/sockio.h> for the SIOCGIFCONF ioctl - this switches
to using a proper configure check for that header rather than just assuming
it'll be there if we can't find SIOCGIFCONF.  Should be more robust.

include/defs.h.in

@@ -279,6 +279,9 @@
 /* Define to 1 if you have the <sys/socket.h> header file. */
 #undef HAVE_SYS_SOCKET_H
 
+/* Define to 1 if you have the <sys/sockio.h> header file. */
+#undef HAVE_SYS_SOCKIO_H
+
 /* Define to 1 if you have the <sys/soundcard.h> header file. */
 #undef HAVE_SYS_SOUNDCARD_H