Clean up something RENEWAL_CAST

* Moved additive bonuses (reducing/increasing) to first calculation before the rate adjustment
* Corrected bFixedCastrate, shouldn't be stacked. Example between Puente_Robe (15012) is -3% and +10 Rafini_Staff (1649) is -10%, only -10% will be used not -13%.
* Reversed some value assignment, `-=` to `+=` and other part that affected by this changes.
* Also as follow up c3e488e & 4f4d8fe, fixed `bonus2 bFixedCastrate,"sk",rate;`
* Corrected `bFixedCastrate` for Krieger_Knuckle2 (1827) only for skill MO_EXTREMITYFIST
* Moved default the 20% of fixed cast rate to conf/battle/skill.conf `default_fixed_castrate`

Signed-off-by: Cydh Ramdh <house.bad@gmail.com>