When rematching, some users have a convention that e.g. lowest scoring
player in the "parent" game goes first. So allow that, providing the
choice on each rematch until a default has been chosen. Support
changing that default in a new prefs setting.
The place I chose to enforce the order was on the host as invitees are
registering and being assigned slots. But by then there's no longer
any connection to the game that was rematched, e.g. to use its
scores. So during the rematched game creation process I create and
store with the new game the necessary ordering information. For the
3-and-4 device case, it was also necessary to tweak the information
about other guests that the host sends guests (added during earlier
work on rematching.)
