socket: introduce SELinuxLabelViaNet option

This makes possible to spawn service instances triggered by socket with
MLS/MCS SELinux labels which are created based on information provided by
connected peer.

Implementation of label_get_child_label derived from xinetd.

Reviewed-by: Paul Moore <pmoore@redhat.com>

man/systemd.socket.xml

@@ -675,6 +675,17 @@
                                 for details.</para></listitem>
                         </varlistentry>
 
+                        <varlistentry>
+                          <term><varname>SELinuxLabelViaNet=</varname></term>
+                          <listitem><para>Takes a boolean
+                          value. Controls whether systemd attempts to figure out
+                          SELinux label used for instantiated service from
+                          information handed by peer over the
+                          network. Configuration option has effect only
+                          on sockets with <literal>Accept=</literal>
+                          mode set to <literal>yes</literal>.</para></listitem>
+                        </varlistentry>
+
                         <varlistentry>
                                 <term><varname>PipeSize=</varname></term>
                                 <listitem><para>Takes a size in