* PHP-8.0:
  Fix #80889: amendment

* PHP-7.4:
  Fix #80889: amendment

`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.

* PHP-8.0:
  Fix #80889: Cannot set save handler when save_handler is invalid

* PHP-7.4:
  Fix #80889: Cannot set save handler when save_handler is invalid

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.

* PHP-8.0:
  Fix #80774: session_name() problem with backslash

* PHP-7.4:
  Fix #80774: session_name() problem with backslash

Since we do no longer URL decode cookie names[1], we must not URL
encode the session name.  We need to prevent broken Set-Cookie headers,
by rejecting names which contain invalid characters.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=6559fe912661ca5ce5f0eeeb591d928451428ed0>

Closes GH-6711.

This deprecates passing null to non-nullable scale arguments of
internal functions, with the eventual goal of making the behavior
consistent with userland functions, where null is never accepted
for non-nullable arguments.

This change is expected to cause quite a lot of fallout. In most
cases, calling code should be adjusted to avoid passing null. In
some cases, PHP should be adjusted to make some function arguments
nullable. I have already fixed a number of functions before landing
this, but feel free to file a bug if you encounter a function that
doesn't accept null, but probably should. (The rule of thumb for
this to be applicable is that the function must have special behavior
for 0 or "", which is distinct from the natural behavior of the
parameter.)

RFC: https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

Closes GH-6475.