Random walking, monster target dropping
- Implemented official behavior for random walking of monsters (bugreport:9340)
  * Updated the interval between walks from 3-6 seconds to 4-5 seconds
  * Added a define MIN_RANDOMWALKTIME that is now used anywhere the interval is applied so it's easier to change
  * Monsters will no longer attempt to walk to the cell they are currently standing on (note: still requires a proper "no cell stacking" implementation)
  * Monsters will only target a cell within a 15x15 area around them
- Moved the "mob_ai" 0x008 configuration setting to mob_unlocktarget, so it applies to all cases of monsters unlocking targets
- Cleaned up the unit_stop_attack and unit_stop_stepaction functions
  * unit_stop_attack now makes sure that the target is always set to 0, even if no attack timer currently exists
  * replaced several calls of unit_set_target that set target to 0 with unit_stop_attack, this is important because the attack timer should always be deleted at the same time the target is set to 0
  * this also fixes the problem that caused monsters to sometimes drop their target after an attack