Implement memcpy_far and checksum_far, and replace _fl variants.

The "flat" mode variants work in real mode, but will not work in
    protected mode.  So, replace with versions that take explicit
    segments.
diff --git a/src/boot.c b/src/boot.c
index 142544b..1f0647a 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -40,7 +40,7 @@
     if (type == 4 && description_fl != 0) {
         char description[33];
         /* first 32 bytes are significant */
-        memcpy_fl(MAKE_FLATPTR(GET_SEG(SS), description), description_fl, 32);
+        memcpy(description, description_fl, 32);
         /* terminate string */
         description[32] = 0;
         printf(" [%.s]", description);