Fix GN_WALLOFTHORN to UNT_FIREWALL transition

[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

Wall of Thorn (GN_WALLOFTHORN) is supposed to burn into a Fire Wall (UNT_FIREWALL) when hit by fire damage, but the resulting fire never actually dealt damage:

• GN_WALLOFTHORN's Unit.Interval was -1 in both db/re/skill_db.conf and db/pre-re/skill_db.conf. Wall of Thorn itself has no periodic on-place tick, so this was fine for its own state, but the morphed Fire Wall unit inherits this same interval. With interval == -1, skill_unit_onplace_timer hits its early-exit guard and logs "interval error" instead of ticking, so the fire never dealt damage for its whole 3-second lifetime. Changed to 300.
• The base Wall of Thorn unit was also missing its 20-hit break limit: val2 held an unused caster ID instead of a hit counter. It now starts at 20 and is decremented on each weapon hit in battle.c (mirroring the existing HT_BLASTMINE hook in the same block), deleting the unit once depleted. skill_unit_timer_sub's expiry check was updated to also break the wall when val2 reaches 0, not just on HP depletion.

Issues addressed: #1118