Further Overthrow Ruler fixes

-"Usurpation" block will now only usurp if any of the rebels has a claim
on the title, and otherwise will abdicate to the rebel leader's
favourite heir (as opposed to the abdication block, which will abdicate
to the primary heir).
-Rulers who are forced to abdicate by internal rebels will now get an
ultimatum from any remaining rebels if they ever re-inherit.