More config.c clean-ups and added some plumbing for the future ZIP and MO image history.

src/include/86box/mo.h

@@ -27,6 +27,8 @@
 
 #define MO_TIME  10.0
 
+#define MO_IMAGE_HISTORY         4
+
 typedef struct mo_type_t {
     uint32_t sectors;
     uint16_t bytes_per_sector;
@@ -113,6 +115,8 @@ typedef struct mo_drive_t {
     char image_path[1024];
     char prev_image_path[1024];
 
+    char *image_history[MO_IMAGE_HISTORY];
+
     uint32_t type;
     uint32_t medium_size;
     uint32_t base;