Minor cleanups.

Improve some debugging messages.
Minor code clean ups.
Improve apmbios entry points (still untested).
diff --git a/src/disk.c b/src/disk.c
index 129b058..b7d004d 100644
--- a/src/disk.c
+++ b/src/disk.c
@@ -349,7 +349,7 @@
     regs->cx = lba >> 16;
     regs->dx = lba & 0xffff;
 
-    disk_ret(regs, 0);
+    disk_ret(regs, DISK_RET_SUCCESS);
     regs->ah = 3; // hard disk accessible
 }