Fix Genetic Sling Item Banana Bomb chance/duration and Mysterious Powder self-debuff

[MrKeiKun]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

GN_SLINGITEM / GN_SLINGITEM_RANGEMELEEATK (Genetic's Sling Item skill) had several bugs affecting Banana Bomb and Mysterious Powder:

• Banana Bomb's sit-down chance formula had the target's AGI term inverted (+ tstatus->agi/4 instead of - tstatus->agi/4), making high-AGI targets more likely to be knocked down instead of less.
• The sit-down duration and the LUK-drain duration used hardcoded placeholder values (1000 and 60000) instead of the job-level-scaled/official durations, and didn't match their own inline comments.
• Mysterious Powder (and other throwable items) could be applied to the caster instead of the target when there was no valid player target (e.g. against a mob), turning a debuff item into a self-debuff.
• Banana Bomb's damage skill ratio was lumped in with Coconut/Pineapple Bomb (+800) instead of using its own official value (+777), and the block was missing the renewal level modifier (RE_LVL_DMOD) applied to other Sling Item ammo.

Issues addressed: #616