Extend the random map river algorithm (rP20185) to allow arbitrary start and end points.

Reverse engineer and cleanup obfuscated Rivers map code, refs rP11137.
Use vector algebra to replace magic equations and express geometric intend.
Fix seed typo by removing river curve duplicate.

On the Rivers map, use areAllies directly instead of copying the diplomacies to a 2D array called isRiver.
Use modulo operator instead of an if-statement with duplication to determine the neighbor of the last player, refs rP11174.