Exclude leader's location from the castle

May slightly speed up search, but more importantly is needed to
consider recruiting from a hex the leader is not currently on
Start of function to prerecruit units

Idea is to collect a set of units that should be recruited at another
castle and make sure that gold exists to recruit them after recruiting
at the current location. If the leader never tries recruiting at the
other location during the turn, the prerecruited units will be discarded