! LuaSynced*: slightly change {Get,Set}ProjectileTarget signatures

  GetProjectileTarget now returns a number instead of a string to indicate target-type
  GetProjectileTarget now always returns two arguments (if given projectileID is valid)
  SetProjectileTarget now takes a number instead of a string to indicate target-type

    GetProjectileTarget(number projectileID) --> nil | [number targetType, number targetID | table targetPos = {[1] = x, ...}]
    SetProjectileTarget(number projectileID, [number targetID [, number targetType]]  |  [number x, number y, number z]) --> nil | boolean

  where targetType is one of {string.byte('g') = GROUND, string.byte('u') = UNIT, string.byte('f') = FEATURE, string.byte('p') = PROJECTILE}