* PHP-8.0:
  Fix #73533: Invalid memory access in php_libxml_xmlCheckUTF8

* PHP-7.4:
  Fix #73533: Invalid memory access in php_libxml_xmlCheckUTF8

A string passed to `php_libxml_xmlCheckUTF8()` may be longer than
1<<31-1 bytes, so we're better using a `size_t`.

Closes GH-6802.

* PHP-8.0:
  Fix #51903: simplexml_load_file() doesn't use HTTP headers

* PHP-7.4:
  Fix #51903: simplexml_load_file() doesn't use HTTP headers

The `encoding` attribute of the XML declaration is optional; it is good
practice to use external encoding information where available if it is
missing.  Thus, we check for `charset` info of `Content-Type` headers,
and see whether the encoding is supported.

We cater to trailing parameters and quoted-strings, but not to escaped
backslashes and quotes in quoted-strings, since no known character
encoding contains these anyway.

Co-authored-by: Michael Wallner <mike@php.net>

Closes GH-6747.

Closes GH-6684

Related to GH-6701

Closes GH-6675