nmbug-status: Convert from XHTML 1.0 to HTML 5

nmbug-status: Convert from XHTML 1.0 to HTML 5

HTML 5 for the win :).  I also de-namespaced the language; the HTML 5
spec allows a vestigial xml:lang attribute, but it's a no-op [1], so I
stripped it.

This shouldn't break anything at tethera, which already serves the
status as text/html:

  $ wget -S http://nmbug.tethera.net/status/
  --2014-02-02 21:20:39--  http://nmbug.tethera.net/status/
  Resolving nmbug.tethera.net... 87.98.215.224
  Connecting to nmbug.tethera.net|87.98.215.224|:80... connected.
  HTTP request sent, awaiting response...
    HTTP/1.1 200 OK
    Vary: Accept-Encoding
    Content-Type: text/html
  ...

This also matches the Content-Type in the generated HTML's http-equiv
meta.

[1]: http://www.w3.org/TR/html5/dom.html#the-lang-and-xml:lang-attributes