Use ZPP callable check for spl_autoload_register.

This makes it always throw a TypeError, moreover this makes the
error message consistent.

Added a warning mentioning that the s

Use ZPP callable check for spl_autoload_register.

This makes it always throw a TypeError, moreover this makes the
error message consistent.

Added a warning mentioning that the second parameter is now ignored
when passed false.

Closes GH-5301

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