Try again to enable the idle call.

DF is undefined on entry, so make sure we clear it at all entry points
where it could possibly matter.

- save and restore all the DOS vectors around COMBOOT invocations.
- handle INT 28h and INT 29h, and IRET on INT 2Dh, 2Fh, 33h.

At least one laptop exists which calls INT 2Fh from inside the BIOS!
So just point INT 2Fh to IRET, and hope that's good enough.

Fix a set of bogus calls to writestr_early instead of writestr.

Mechanically rename the writestr functions:

	writestr   -> writestr_early
	cwritestr  -> writestr

... to reflect which function is more appropriate to call in normal
circumstances.

Print the interrupt number, AX, and CS:IP of the offending system
call.

Enable idle calls on select platforms, based on device ID.
In particular, certain Broadcom chips need them to avoid FIFO stalls.