Fix HTML parser with threads and --without-legacy

If the legacy functions are disabled, the default "V1" HTML SAX handler
isn't initialized in threads other than the main thread.
htmlInitParserCtxt would later use the empty V1 SAX handler, resulting
in NULL documents.

Change htmlInitParserCtxt to initialize the HTML SAX handler by calling
xmlSAX2InitHtmlDefaultSAXHandler. This removes the ability to change the
default handler but is more in line with the XML parser which
initializes the SAX handler by calling xmlSAXVersion, ignoring the V1
default handler.

Fixes #399.