This remained undetected for a long time as HeaderCheck is disabled by
default. This commit makes the following file compile again:
// file: compile_me.cpp
#include <LibDNS/Question.h>
// That's it, this was enough to cause a compilation error.
Likewise for most other files touched by this commit.
Currently this can parse XML and resolve external resources/references,
and read a DTD (but not apply or verify its rules).
That's good enough for _most_ XHTML documents as the HTML 5 spec
enforces its own rules about document well-formedness, and does not make
use of XML DTDs (aside from a list of predefined entities).
An accompanying `xml` utility is provided that can read and dump XML
documents, and can also run the XML conformance test suite.
