support noipv6 being set externally even when there is perfectly good

IPv6 support.

socket/socket_tcp6.c

@@ -12,7 +12,7 @@
 int socket_tcp6(void) {
   int s;
 #if defined(LIBC_HAS_IP6) && defined(SOCK_NONBLOCK)
-  if ((s=socket(PF_INET6,SOCK_STREAM|SOCK_NONBLOCK,IPPROTO_TCP))>-1 || errno!=EINVAL) return s;
+  if ((s=socket(noipv6?PF_INET:PF_INET6,SOCK_STREAM|SOCK_NONBLOCK,IPPROTO_TCP))>-1 || errno!=EINVAL) return s;
 #endif
   s=socket_tcp6b();
   if (s==-1) return -1;