(I_NONE, I_CIVIL, I_MILIT, I_SHELL, I_GUN, I_PETROL, I_IRON, I_DUST,

(I_NONE, I_CIVIL, I_MILIT, I_SHELL, I_GUN, I_PETROL, I_IRON, I_DUST,
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.