[trac1012] add "id" to the xml varlistentry which results in HTML anchor

stephen suggested in jabber
stephen okayed in jabber

example:
http://bind10.isc.org/docs/bind10-messages.html#DATASRC_QUERY_REF_FAIL

tools/system_messages.py

@@ -115,7 +115,7 @@ SEC_HEADER="""<?xml version="1.0" encoding="UTF-8"?>
 # This is output once for each message.  The string contains substitution
 # tokens: $I is replaced by the message identification, $T by the message text,
 # and $D by the message description.
-SEC_MESSAGE = """<varlistentry>
+SEC_MESSAGE = """<varlistentry id="$I">
 <term>$I, $T</term>
 <listitem><para>
 $D