socket: Allow selection of TCP Congestion Avoidance algorithm to socket

Hi,

attached path extends socket configurables with another
knob - TCP Congestion Avoidance selection. Linux implements
handful of those, useful in various situations. For example,
TCP Low Priority may be used by FTP service to gracefully
yield bandwidth for more important TCP/IP streams.

Until recently TCP_CONGESTION was Linux-specific, recently
FreeBSD 8 and OpenSolaris gained compatible support.

man/systemd.socket.xml

@@ -437,6 +437,18 @@
                                 address. Defaults to <option>false</option>.</para></listitem>
                         </varlistentry>
 
+                        <varlistentry>
+                                <term><varname>TCPCongestion=</varname></term>
+                                <listitem><para>Takes a string
+                                value. Controls the TCP congestion
+                                algorithm used by this socket. Should
+                                be one of "westwood", "veno", "cubic",
+                                "lp" or any other available algorithm
+                                supported by the IP stack. This
+                                setting applies only to stream
+                                sockets.</para></listitem>
+                        </varlistentry>
+
                         <varlistentry>
                                 <term><varname>ExecStartPre=</varname></term>
                                 <term><varname>ExecStartPost=</varname></term>