Reduce stack usage for ISRs; minor fixes.

Don't back up all registers on isr handlers - they don't read/modify
    them.  This saves stack space.
extended_bios_data_area_s must be packed to match ebda spec.
Enable irqs on int 08 - follows old bochs bios code.
Fix bug in int 76 -- should clear disk_interrupt_flag not
Make sure we alert in disk_ret on failure case.
int 18/19 entry points need to setup cld/%ds too.
asm in handle_1587 clobbers flags - note that in clobber list.
diff --git a/TODO b/TODO
index a32c4b7..603d763 100644
--- a/TODO
+++ b/TODO
@@ -1,5 +1,8 @@
 Audit all sti/cli calls.
+Audit statements where a 32bit intermediary changes meaning of a 16bit
 Look into ways to reduce stack usage.  For example, %esp, %ebp, %esi
 (high bits), %edi (high bits) are already restored by the C code if
 they're changed - they probably don't need to be backed up on entry.