Replace common segment/offset pairs with struct segoff_s.

Introduce 'struct segoff_s' to more places.
diff --git a/src/asm-offsets.c b/src/asm-offsets.c
index c324151..5035cef 100644
--- a/src/asm-offsets.c
+++ b/src/asm-offsets.c
@@ -20,7 +20,7 @@
     OFFSET(BREGS_esi, bregs, esi);
     OFFSET(BREGS_edi, bregs, edi);
     OFFSET(BREGS_flags, bregs, flags);
-    OFFSET(BREGS_ip, bregs, ip);
+    OFFSET(BREGS_code, bregs, code);
 
     COMMENT("BDA");
     OFFSET(BDA_ebda_seg, bios_data_area_s, ebda_seg);