Simplify and lock down priv_open():

Simplify and lock down priv_open():
 * kill the 'mode' argument
 * fail if passed any flags other than O_ACCMODE OR O_NONBLOCK
 * paranoia: mask O_CREAT when calling open() with only two arguments
 * instead of using ioctl(FIONBIO) after the fact, pass O_NONBLOCK to
   priv_open()

"good start" deraadt@
ok yasuoka@