Make IPv6 flow labels work correctly where they didn't before. (#694)

This can happen with multiple parallel streams or with two single
stream tests run close together in time.

The fix is to allow flow labels to be reused by multiple sockets
simultaneously or within their timeout.

Submitted by @cygnus2048 and fixes #689.

src/iperf_tcp.c

@@ -1,5 +1,5 @@
 /*
- * iperf, Copyright (c) 2014, 2016, 2017, The Regents of the University of
+ * iperf, Copyright (c) 2014-2018, The Regents of the University of
  * California, through Lawrence Berkeley National Laboratory (subject
  * to receipt of any required approvals from the U.S. Dept. of
  * Energy).  All rights reserved.
@@ -509,7 +509,7 @@ iperf_tcp_connect(struct iperf_test *test)
             freq->flr_label = htonl(test->settings->flowlabel & IPV6_FLOWINFO_FLOWLABEL);
             freq->flr_action = IPV6_FL_A_GET;
             freq->flr_flags = IPV6_FL_F_CREATE;
-            freq->flr_share = IPV6_FL_F_CREATE | IPV6_FL_S_EXCL;
+            freq->flr_share = IPV6_FL_S_ANY;
             memcpy(&freq->flr_dst, &sa6P->sin6_addr, 16);
 
             if (setsockopt(s, IPPROTO_IPV6, IPV6_FLOWLABEL_MGR, freq, freq_len) < 0) {