Restart after disconnect

[jetholloway]

Hi Tim,

(Note: this is my second pull request made today for grabserial, so it might make sense to look at the other one first. The other one (#38 ) is also much simpler -- some minor bug fixes.)

This pull request allows grabserial to handle disconnections of the serial device. I.e. if you have a USB UART, and you unplug it from the USB port.

• Tested on Linux, Python 2 and 3. Tested on Windows, Python 2. Tested that it still restarts with -q and -e options. Tested that you can quit using Ctrl-C without causing an error when UART is disconnected or connected. Tested that it works when the program starts with the UART disconnected, and then it is plugged in afterwards.
• Note that this "restart on disconnect" feature requires that the user specify the "-S" option, otherwise grabserial will quit when it fails to find the serial port device, because it won't exist when you unplug the UART.
• Do you want me to add another command-line option for restarting upon disconnects, or just leave it as the default behaviour (it still requires "-S")? Perhaps the "-a" option could be modified to allow things like "-a=eqd", where 'e' means restart on timeout, 'q' for restart on quitpat, and 'd' for restart on disconnect.
• Also, the use of "restart on disconnect" requires stable device names. I.e. when you disconnect /dev/ttyUSB2, and re-connect it, it must appear as /dev/ttyUSB2. Stable device names can be achieved with udev rules. Do you want me to add some text to explain these things? Maybe in the README, because it would be a bit too much text to put in the 'usage' string.
• Let me know if you think anything in the code should be changed.

Example usage: grabserial -S -a ...

[tbird20d]

This looks like a nice feature. Thanks.

As for suggested enhancements - it might be good to add something explaining how this works in READM.md.

-- Tim