[1534] Use the other methods of setting it as fallback

Meaning if it works, don't try the others. Also, a little comment in
test was added.

src/bin/sockcreator/sockcreator.cc

@@ -178,7 +178,7 @@ mtu(int fd) {
     if (setsockopt(fd, IPPROTO_IPV6, IPV6_USE_MIN_MTU, &on, sizeof(on)) < 0) {
         return (-2);
     }
-#endif
+#else // Try the following as fallback
 #ifdef IPV6_MTU
     // Use minimum MTU on systems that don't have the IPV6_USE_MIN_MTU
     const int mtu = 1280;
@@ -195,6 +195,7 @@ mtu(int fd) {
         return (-2);
     }
 #endif
+#endif
     return (fd);
 }
 

src/bin/sockcreator/tests/sockcreator_tests.cc

@@ -125,7 +125,7 @@ void addressFamilySpecificCheck(const sockaddr_in6*, const int socknum,
         EXPECT_EQ(getsockopt(socknum, IPPROTO_IPV6, IPV6_USE_MIN_MTU, &options,
                              &len)) << strerror(errno);
         EXPECT_NE(0, options);
-#endif
+#else
         // We do not check for the IPV6_MTU, because while setting works (eg.
         // the packets are fragmented correctly), the getting does not. If
         // we try to getsockopt it, an error complaining it can't be accessed
@@ -138,6 +138,7 @@ void addressFamilySpecificCheck(const sockaddr_in6*, const int socknum,
                                 &options, &len)) << strerror(errno);
         EXPECT_EQ(IPV6_PMTUDISC_DONT, options);
 #endif
+#endif
     }
 }
 
@@ -204,6 +205,9 @@ TEST(get_sock, tcp6_create) {
 
 bool close_called(false);
 
+// You can use it as a close mockup. If you care about checking if it was really
+// called, you can use the close_called variable. But set it to false before the
+// test.
 int closeCall(int socket) {
     close(socket);
     close_called = true;