Replace common segment/offset pairs with struct segoff_s.

Introduce 'struct segoff_s' to more places.
diff --git a/src/boot.c b/src/boot.c
index 9a08d26..af2a893 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -323,8 +323,7 @@
 
     struct bregs br;
     memset(&br, 0, sizeof(br));
-    br.ip = bootip;
-    br.cs = bootseg;
+    br.code = SEGOFF(bootseg, bootip);
     // Set the magic number in ax and the boot drive in dl.
     br.dl = bootdrv;
     br.ax = 0xaa55;