Introduce MODESEGMENT define; rename VISIBLE32 to VISIBLE32FLAT.

Prepare for support of segmented 32bit code.
Add new MODESEGMENT definition, and clarify existing 32bit mode defs.
diff --git a/src/boot.c b/src/boot.c
index 05de715..c427c00 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -486,7 +486,7 @@
 }
 
 // Boot Failure recovery: try the next device.
-void VISIBLE32
+void VISIBLE32FLAT
 handle_18()
 {
     debug_serial_setup();
@@ -498,7 +498,7 @@
 }
 
 // INT 19h Boot Load Service Entry Point
-void VISIBLE32
+void VISIBLE32FLAT
 handle_19()
 {
     debug_serial_setup();