More gracefully handle the case where a congestion control

algorithm isn't available on the server.  This can happen
if the client and server machines have different sets of
congestion control algorithms loaded in kernel modules, etc.
If the requested algorithm isn't available on the server, then
print a warning on the server side, but otherwise continue to
run the test.

Towards #549.

src/iperf_server_api.c

@@ -1,5 +1,5 @@
 /*
- * iperf, Copyright (c) 2014, 2015, 2016, The Regents of the University of
+ * iperf, Copyright (c) 2014, 2015, 2016, 2017, 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.
@@ -518,10 +518,24 @@ iperf_run_server(struct iperf_test *test)
 		    if (test->protocol->id == Ptcp) {
 			if (test->congestion) {
 			    if (setsockopt(s, IPPROTO_TCP, TCP_CONGESTION, test->congestion, strlen(test->congestion)) < 0) {
-				close(s);
-				cleanup_server(test);
-				i_errno = IESETCONGESTION;
-				return -1;
+				/*
+				 * ENOENT means we tried to set the
+				 * congestion algorithm but the algorithm
+				 * specified doesn't exist.  This can happen
+				 * if the client and server have different
+				 * congestion algorithms available.  In this
+				 * case, print a warning, but otherwise
+				 * continue.
+				 */
+				if (errno == ENOENT) {
+				    warning("TCP congestion control algorithm not supported");
+				}
+				else {
+				    close(s);
+				    cleanup_server(test);
+				    i_errno = IESETCONGESTION;
+				    return -1;
+				}
 			    } 
 			}
 			{