Updates the settings table as well as $modSettings.
.. only does one at a time if $update is true.
- updates both the settings table and $modSettings array.
- all of changeArray's indexes and values are assumed to have escaped apostrophes (')!
- if a variable is already set to what you want to change it to, that
variable will be skipped over; it would be unnecessary to reset.
- When use_update is true, UPDATEs will be used instead of REPLACE.
- when use_update is true, the value can be true or false to increment
or decrement it, respectively.