[Freeciv-Dev] (PR#9380) new client function city_can_buy

["Jason Dorje Short" <jdorje@xxxxxxxxxxx>]

<URL: http://rt.freeciv.org/Ticket/Display.html?id=9380 >

This patch adds a new function city_can_buy(), and adds capitalization 
(B_CAPITAL) support to impr_buy_gold_cost().

city_can_buy() is in citydlg_common, and can be called by any citydlg 
code to see if the city can buy right now.  Cities can't buy if the 
client is an observer, if they've already bought this turn, or if the 
production is unbuyable (already complete or capitalization).  The 
function is used in citydlg code throughout all the clients.  This 
shortens the amount of code needed, improves the behavior (IMO), and 
removes references to B_CAPITAL.

Correspondingly, impr_buy_gold_cost is adjusted to return 0 for 
capitalization.  Previously it would always return 1998.  Thus 
city_buy_cost would return 3996 instead of 0.

jason