digitalPin: do not export if the /sys/class/gpio/... file already exists

[SpComb]

Change digitalPin to allow the use of an already exported gpio pin, avoiding the write /sys/class/gpio/export: device or resource busy errors that currently result. This kind of situation is easy to end up in if you e.g. kill the process and restart it, without giving it a chance to unexport its pins.

This should suffice to handle the issues mentioned in #30 and #44.

This also helps to deal with the open /sys/class/gpio/gpio21/direction: permission denied races in #52, by allowing us to use a helper script to pre-export the gpio pins and wait for udev to settle.

[tve]

I've taken a slightly different approach in #67, which is to ignore the error if the pin has already been exported. This way it's only one system call.

[SpComb]

That sounds like it would also work just as well.

[wiless]

I used to workaround catching os signal, and unexporting in init through cmd.exec

[tve]

@wiless I don't understand. The fix in #69 is 091d6f5, I'm not sure what signals or cmd.exec have to do with that...

[wiless]

@tve I meant to say was my workaround to handle unexported GPIO issue

• In the beginning of my application , I monitor the interrupts from os/signal and "unexport" before terminating application.
• Also in the init() I attempt to unexport the GPIO through calling echo 1 > /sys/class/gpio.. through cmd.exec. This ensures that the error is avoided.

[tve]

I'm pretty convinced that this is taken care of by #69. Please reopen with an example if it's not.