kernel - Fix live lock in vfs_conf.c mountroot>

kernel - Fix live lock in vfs_conf.c mountroot>

* The mountroot> prompt calls cngetc() to process user input.  However, this
  function hard loops and can prevent other kernel threads from running on
  the current cpu.

* Rearrange the code to use cncheckc() and a 1/25 second tsleep().

* Fix a bug in the syscons code where NOKEY was not being properly returned
  as documented.  Modify all use cases to handle NOKEY.  This allows us to
  differentiate between a keyboard present but not key pressed and a keyboard
  not present.

* Pull the automatic polling mode code out of cncheckc() (or more precisely,
  out of sccncheckc()) and add a new cnpoll() API function to set it manually.

  This fixes issues in vfs_conf when normal keyboard processing interrupts
  are operational and cncheckc() is used with a tsleep() delay.  The normal
  processing interrupt wound up eating the keystrokes so the cncheckc()
  basically always failed.

  cncheckc() in general also always had a small window of opportunity where
  a keystroke could be lost due loops on it.

* Call cnpoll() in various places, such as when entering the debugger,
  asking for input in vfs_conf, and a few other places.