Fix XHTML DOCTYPE

This patch fixes incorrect System Identifier part of the DOCTYPE reported as a warning by the W3C Validator. It also fixes a few case of missing `xmlns` in XHTML documents.
See for example [the W3C Validator report for `htdocs/imprint.html`](http://validator.w3.org/check?verbose=1&uri=http%3A%2F%2Fpause.perl.org%2Fimprint.html) that show both issues.