Simplify take_move_in_mob()

Simplify take_move_in_mob()

Actually, this isn't just simplification.  When mobility gain per
update was configured to be greater than 128, mobility could go from 1
to less than -127 when assaulting from a landing ship, and thus
overflow.  Make it saturate at -127.  Note that you can expect plenty
of trouble elsewhere with such a silly configuration.