I_BAR, I_FOOD, I_OIL, I_LCM, I_HCM, I_UW, I_RAD, I_MAX): Turn macros
into enumeration constants.
(i_type): New. Use where appropriate. No functional changes, except
that I_NONE is now catched properly in a few places.
member com_type from mnemo character to item type.
(whichitem): Unused, remove.
(check_market): Use full item name instead of mnemo in telegrams.
(display_mark): Separate arguments for item type and cheapest only.
Cheapest only of specific item type is not implemented.
(rese): Guard against bad com_type. Delete some code that had no
effect.
(genitem_ca, ship_ca, plane_ca, land_ca): Remove selector "sell".
(comstr, trdstr): Members trd_price, com_price have no effect. Use
them instead of trd_maxprice, com_maxprice and remove the latter.
(commodity_ca, trade_ca): Remove selector "maxprice".
