[z1865] Changes to default target selection

* Add a new function: Unit::IsSecondChoiceTarget to evaluate if a target will be selected
* Cleanup a bit related code
* SelectHostileTarget
 - Only call AI()->AttackStart for new targets
 - Remove exception for top-most taunter
* Fix a few minor bugs related to selectNextVictim

Signed-off-by: Schmoozerd <schmoozerd@scriptdev2.com>

(based on commit 51d2766)

(cleaned version of 29cf8ea)

(based on commit [s1429] - a733469)