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:

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5545

Differential Revision: https://secure.phabricator.com/D9811