Fix a problem when getting snd_cwnd on FreeBSD.

On FreeBSD, unlike Linux (and NetBSD?) snd_cwnd is expressed in
octets instead of segments.  Hilarity ensued when we erroneously
multiplied by snd_mss and integer overflows occureed.

Possible fix for #465, #475, #338.  Testing from FreeBSD users
appreciated.

src/tcp_info.c

@@ -135,7 +135,7 @@ get_snd_cwnd(struct iperf_interval_results *irp)
 #if defined(linux) && defined(TCP_MD5SIG)
     return irp->tcpInfo.tcpi_snd_cwnd * irp->tcpInfo.tcpi_snd_mss;
 #elif defined(__FreeBSD__) && __FreeBSD_version >= 600000
-    return irp->tcpInfo.tcpi_snd_cwnd * irp->tcpInfo.tcpi_snd_mss;
+    return irp->tcpInfo.tcpi_snd_cwnd;
 #elif defined(__NetBSD__) && defined(TCP_INFO)
     return irp->tcpInfo.tcpi_snd_cwnd * irp->tcpInfo.tcpi_snd_mss;
 #else