Bug 34041: (follow-up) escape double dashes to prevent issues

Bug 34041: (follow-up) escape double dashes to prevent issues

Having double dashes inside a commmented block is not valid XML. This
patch restores it, with an added message explaining it

To test:
1. Run:
   $ xmllint etc/z3950/config.xml
=> FAIL: You get:
etc/z3950/config.xml:5: parser error : Double hyphen within comment: <!--
  <config>
      <z3950_responder_options>
      <z3950_responder_options>--add-item-status k -t 5</z3950_responder_options
2. Apply this patch
3. Repeat 1
=> SUCCESS: All good!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>