Harmonized ststus texts and and added status on wrong MIME type.
Lifted restrictions on accepted MIME type.

The "normal" web is moving forward quickly reducing the interoperability of the Contiki web browser to nearly zero. The Mobile Web fits the capabilities of the Contiki web browser much better. Modern smartphones don't need the Mobile Web anymore but there are large areas in world with rather low end mobile phones and limited mobile bandwidth where the Mobile Web will be necessary for quite some time.

From that perspective it is reasonable to increase the Contiki web browser's interoperability with the Mobie Web - namely WAP 2.0 aka XHTML MP. XHTML MP is delivered as MIME types 'application/vnd.wap.xhtml+xml' or 'application/xhtml+xml'. Therefore we (try to) parse the document if the MIME type contains the substring 'html' (which is true 'text/html' too).
Added newline on </div>.

The tag <div> (in contrast to the tag <span>) is normally used to denote content placed on a line by its own. So it makes sense to trigger a newline when </div> is processed.
Added support for self closing tags.

XHTML requires self closing tags to be used for empty tags, so we need to recognize them.
Made sure that HTML form values get initialized properly.
Removed useless register keywords.

Modern compilers (especially GCC) ignore the register keyword anyway and the latest cc65 snapshot generates actually larger code with the register keyword at the locations in question.