Cast both value and units to int for configduration write_setting

The duration config type takes two values, a value and a unit.

The unit is generally always something that can be cast to int.

However, the value, which is provided by the user - might not be. If no value is entered, or a value which does not cast to an int - then php performs a string*string operation instead of int*int.

For php before php8 - this simply resulted in a Warning - and the default validation was able to confirm it wasn't set correctly and prompt the user for a better value.

From php8 however, string*string opts now raise a TypeError exception - so the user instead sees "Exception - Unsupported operand types: string * string".

The fix for this is rather trivial - rather casting the result of u * v to an int, cast each value separately to an int before multiplying them.