Freeciv

Continuation of #1375. The following methods are needed: * (City):settle_specialist(Specialist s, int num = 1) - adds a specialist or several; for normal specialist, also increases city size. Respects s's reqs. Returns number of specialists added. * (City):num_specialists(Specialist s?) - counts specialists of given type in the city, without parameter, counts all specialists (normal and super). Maybe add functions for counting all normal and all super specialists? * (City):reduce_specialist(Specialist s, int num = 1) - for superspecialist, just removes them, for normal specialist, toggles them cyclically (does not place workers or change city size), returns number of toggled/removed ones. * (City):can_use(Specialist s) - checks reqs (specialist can be still not addable due to 255 limit) * Player:can_use(Specialist s) - checks Player+-ranged reqs. Maybe rename these functions to sth more specific like can_employ but actually we have polymorphism.