A little cleanup for Pet DB

* Separated pet_db for Renewal & Pre-Renewal to avoid non-existant monster in different mode.
* Cleaned script command in pet_db.txt file, just point it to doc/script_commands.txt. Read them there!
* Separated damage value for petskillattack2 to its own var 'damage', that previously use 'lv' var to store the value.
* Added config 'pet_ignore_infinite_def' in conf/battle/pet.conf,
  * If disabled (val:"no"), fixed damage from pet script 'petskillattack2' will be adjusted to 1. Example, just like player attacks Shining Plant.
  * Enabled by default. Previously, fixed damage ignore the infinite defense, that's why it's enabled by default.
  * TODO: Correcting the logic and maybe also confirm the correct behavior!
* Added validation for pet script commands,
  * Skill for petskillattack, petskillattack2, & petskillsupport
  * SC range for petrecovery.
* Cleaned up some source documentation.

Signed-off-by: Cydh Ramdh <house.bad@gmail.com>