Enhance included bios tables.

Moved floppy parameter table to its 0xefc7 location.
Define floppy base table as a struct.
Cleaned up definitions in romlayout.S
Fixed bug in handle_15c0 - wasn't clearing cf.
Clean up some post.c usages of bios tables.
diff --git a/src/boot.c b/src/boot.c
index 8595b7f..19a4929 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -11,6 +11,11 @@
 #include "cmos.h" // inb_cmos
 #include "ata.h" // ata_detect
+// We need a copy of this string, but we are not actually a PnP BIOS,
+// so make sure it is *not* aligned, so OSes will not see it if they
+// scan.
+char pnp_string[] VISIBLE __attribute__((aligned (2))) = " $PnP";
 // print_boot_device
 //   displays the boot device