ucom console again [PATCH]

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

Hello

I worked a lot with Nick Hudson on ucom console support. It is nice to 
have a public review from time to time, so here is the latest patch.
https://dl.espci.fr/ticket/8941ff7b7888a716cebc9b2f3bc3ddf4

Thanks to Nick, many hacks were removed. Unfortunately, the situation
deteriorated a bit on qemu while it improved on real hardware. Here
is the status so far: 

boot -a and DDB work without a hitch on real hardware.

On Qemu:
- boot -a sometime hang on a prompt. There is a race somewhere, 
  I suspect it does not happen on real hardware because Qemu 
  serial emulation is much faster than a real serial port. 
- getty fails to start because uftdi.c fails ioctl TIOCSETA with 
  USBD_STALLED. It seems so unrelated to my changes that I wonder
  it I am not hitting an unrelated bug.

Update just for Nick: I finaly implemented console input without
cn_xfer_in. When console is attached, we always schedule input
transfers, regardless of the device being open or not. That makes
polling transitions simplier because there is no need to switch
between console and standard device input xfer.
 
-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost