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.

We need to do that in case a user handler has been set.  However, we
can't do that in `php_rinit_session_globals()` since that function is
called by PHP function `session_destroy()` too, but in that case we
don't want to reset PS(mod).

Closes GH-6795.

`session_set_save_handler()` may be called with callables instead of an
object; we need to cater to that as well.

We also extract a set_user_save_handler_ini() function to avoid code
duplication.

Closes GH-6796.

There is no need to require a (valid) save_handler to be set, when a
user handler is supposed to be set.  We just have to make sure, that
no user handler is already set in this case.

Closes GH-6788.

It is not guaranteed, that the driver inserts only a single NUL byte at
the end of the buffer.  Apparently, there is no way to find out the
actual data length in the buffer after calling `SQLGetData()`, so we
adjust after the next `SQLGetData()` call.

We also prevent PDO::ODBC_ATTR_ASSUME_UTF8 from fetching garbage, by
fetching all chunks with the same C type.

Closes GH-6716.

The compile context is shared between patterns, so we need to set
the character tables unconditionally in case we switched from
a non-C locale to the C locale.

According to the DOM standard, elements may only contain element, text,
processing instruction and comment nodes[1].  It is also specified that
a HierarchyRequestError should be thrown if a document is to be
inserted[2].  We follow that standard, and prevent the use-after-free
this way.

[1] <https://dom.spec.whatwg.org/#node-trees>
[2] <https://dom.spec.whatwg.org/#mutation-algorithms>

Closes GH-6765.