Add an "UnknownSignal" value for the pinout signals

This needs to be added to the API because there are situations where the serial
line state is unknown. The two typical use cases are:

1) The syscall cannot retrieve the state for some reason.
2) The serial port is not open.

This has been (only build!) tested on Linux (Arch) with Qt 4 and then 5.

Change-Id: I471a68b2e335b54d9a94071f08b1b31a6fb0056a
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Return in certain methods if the device is not open

This is necessary for a couple of methods because they require the port being
open for being functional. Hence, if the port is not open, they now return
properly on their own, and set the "NotOpenError" error code .

This has been (only build!) tested on Linux (Arch) with Qt 4 and then 5.

Change-Id: Ifc77bea581dd1832e188fa7562c5c1b15c0532e3
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>