Defines starting with `i' in hexen

• __ICDMUS__ src/i_cdmus.h:5
• __I_HEADER_H__ src/i_header.h:2
• INFRATICS src/h2def.h:583
• INVISTICS src/h2def.h:582
• INVULNTICS src/h2def.h:581
• IRONTICS src/h2def.h:584