(svn r10541) -Fix [FS#1028]: cargo payment rates overflow.

-Fix: cargo payment rates diverge from cost rates making it impossible to make any profit after a certain number of years.
 Both are solved by stopping the inflation after 170 years; there is absolutely no point in continueing the inflation after that as it only makes the game have overflows at some point that cannot be solved; using larger variables only delays the inevitable.

src/economy.h

@@ -11,6 +11,7 @@ void SetPriceBaseMultiplier(uint price, byte factor);
 struct Economy {
 	Money max_loan;         ///< Maximum possible loan
 	Money max_loan_unround; ///< Economy fluctuation status
+	uint16 max_loan_unround_fract; ///< Fraction of the unrounded max loan
 	int fluct;
 	byte interest_rate;     ///< Interest
 	byte infl_amount;       ///< inflation amount