[z1888] Inform AI function of original caster in some cases

Inform the AI of an original caster for JustSummoned and SpellHitTarget.
This should make scripting some hard cases easier.
Remark that _both_ caster and original caster will be informed (if they are different).

Check your scripts that they still behave as expected!

(cleaned version of 613df87)

(based on commit [s1461] - d236d10)