Fix #56: Check TTY before trying to start getty
Fix #56: Check TTY before trying to start getty

This (initial) patch for issue #56 adds a very basic validation of the
device before attempting to start a getty on it.

TODO: Verify a USB/serial device can be plugged in at any point at
      runtime, either automatically start getty or by `initctl reload`

Signed-off-by: Joachim Nilsson <583c295fd7602c168ad814279bbc3894ba65f5d6@gmail.com>

troglobit/finit,troglobit/finit