Update parts of interpreter, that render some (x)HTML

Hi.

This pull-request introduces changes on source code, that renders some (x)HTML to end-user. Affected are:

 - `phpinfo()` core function
 - PHP core error messages
 - tests

I have changed very old XHTML code used by actual interpreter according to [HTML — Living Standard specification](https://html.spec.whatwg.org/multipage/).
`SG(default_charset)`
You'd probably need to do some converting on output for non-utf8. Plus it's hardcoded [here](https://github.com/Kubo2/php-src/blob/012f046550f9fda94545c65d20493cf3d30f29e9/ext/standard/info.c#L71).

(Such note does not belong to the code. Ever.)