core: set $SYSTEMD_EXEC_PID= environment variable for executed commands

It may be useful to detect a command is directly executed by systemd
manager, or indirectly as a child of another process.

man/systemd.exec.xml

@@ -3043,6 +3043,17 @@ StandardInputData=SWNrIHNpdHplIGRhIHVuJyBlc3NlIEtsb3BzLAp1ZmYgZWVtYWwga2xvcHAncy
           </para></listitem>
         </varlistentry>
 
+        <varlistentry>
+          <term><varname>$SYSTEMD_EXEC_PID</varname></term>
+
+          <listitem><para>The PID of the unit process (e.g. process invoked by
+          <varname>ExecStart=</varname>). The child process can use this information to determine
+          whether the process is directly invoked by the service manager or indirectly as a child of
+          another process by comparing this value with the current PID (as similar to the scheme used in
+          <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+          with <varname>$LISTEN_PID</varname> and <varname>$LISTEN_FDS</varname>).</para></listitem>
+        </varlistentry>
+
         <varlistentry>
           <term><varname>$TERM</varname></term>