man: use "=" when referring to configuration file settings

This convention is almost universal in systemd man pages, and makes
it easier to visually parse the docs.

Also fix some markup along the way.

man/machine-id.xml

@@ -119,7 +119,7 @@ id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
     <filename>/etc/machine-id</filename> originates in the
     <filename>/var/lib/dbus/machine-id</filename> file introduced by
     D-Bus. In fact, this latter file might be a symlink to
-    <varname>/etc/machine-id</varname>.</para>
+    <filename>/etc/machine-id</filename>.</para>
   </refsect1>
 
   <refsect1>

man/systemd.service.xml

@@ -287,7 +287,7 @@
         below (see section "Command Lines" below).
         </para>
 
-        <para>When <varname>Type</varname> is not
+        <para>When <varname>Type=</varname> is not
         <option>oneshot</option>, only one command may and must be
         given. When <varname>Type=oneshot</varname> is used, zero or
         more commands may be specified. This can be specified by
@@ -421,7 +421,7 @@
         <varname>ExecStop=</varname> defined, or where the service
         exited unexpectedly. This argument takes multiple command
         lines, following the same scheme as described for
-        <varname>ExecStart</varname>. Use of these settings is
+        <varname>ExecStart=</varname>. Use of these settings is
         optional. Specifier and environment variable substitution is
         supported.</para></listitem>
       </varlistentry>
@@ -486,7 +486,7 @@
         "keep-alive ping"). If the time between two such calls is
         larger than the configured time, then the service is placed in
         a failed state and it will be terminated with
-        <varname>SIGABRT</varname>. By setting
+        <constant>SIGABRT</constant>. By setting
         <varname>Restart=</varname> to <option>on-failure</option> or
         <option>always</option>, the service will be automatically
         restarted. The time configured here will be passed to the
@@ -908,11 +908,11 @@
         <listitem><para>Configure the location of the file containing
         <ulink url="https://www.kernel.org/doc/Documentation/usb/functionfs.txt">functionfs</ulink>
         descriptors. This is is used only when a socket with
-        <varname>ListenUSBFunction</varname> configured wants to
+        <varname>ListenUSBFunction=</varname> configured wants to
         activate this service. Content of this file is writen to
         <filename>ep0</filename> file after it is opened. This is
         required for socket activation using
-        <varname>ListenUSBFunction</varname> (i.e. for passing all
+        <varname>ListenUSBFunction=</varname> (i.e. for passing all
         functionfs endpoints to the service).</para></listitem>
       </varlistentry>
 

man/systemd.socket.xml

@@ -270,8 +270,8 @@
         <varname>ListenFIFO=</varname> directive above. Use this to
         open functionfs endpoint <filename>ep0</filename>. When using
         this option, activated service has to have the
-        <varname>USBFunctionDescriptors</varname> and
+        <varname>USBFunctionDescriptors=</varname> and
-        <varname>USBFunctionStrings</varname> options set.
+        <varname>USBFunctionStrings=</varname> options set.
         </para></listitem>
       </varlistentry>