R1:04d5402e2fdd

Fatal during setup for "mbstring.func_overload"

Summary:
Fixes T5545. We assume `strlen()` returns the number of bytes in a string, which is the normal behavior (and the documented behavior).

There's a config option, `mbstring.func_overload`, which silently calls mb_strlen() instead. This may return some other result, might fail, etc., and there's no way to get the byte length of a string if this option is set.

If this option is set, fatal immediately. Nothing good can ever come of it.

Test Plan…