Core/Handlers: Add inspect distance and target checks

Add some missing server-side checks based on the following:
http://www.wowwiki.com/API_CheckInteractDistance
http://www.wowwiki.com/API_CanInspect

IsValidAttackTarget is used instead of IsHostileTo, because of the sanctuary areas and pvp off cases.
Not a big exploit; you could query your enemy's talents, arena teams, etc.