Unexport digital pins on Close().

[wujiang]

Exported digital pins can be uninitialized.

How to reproduce the problem

package main

import (
        "fmt"

        "github.com/kidoman/embd"
        _ "github.com/wujiang/embd/host/rpi"
)

func main() {
        embd.InitGPIO()
        defer embd.CloseGPIO()
        fmt.Println(embd.SetDirection(12, embd.Out))
}

/tmp $ go build
/tmp $ ./tmp
open /sys/class/gpio/gpio12/direction: permission denied
/tmp $ ./tmp
write /sys/class/gpio/export: device or resource busy
/tmp $ ./tmp
write /sys/class/gpio/export: device or resource busy
/tmp $ ls /sys/class/gpio/
export  gpio12  gpiochip0  unexport

After the change:

/tmp $ ./tmp
open /sys/class/gpio/gpio12/direction: permission denied
/tmp $ ./tmp
open /sys/class/gpio/gpio12/direction: permission denied
/tmp $ ./tmp
open /sys/class/gpio/gpio12/direction: permission denied

[jpillora]

:+1: Just ran into this issue

[tve]

While I understand the problem, it seems to me that the unexporting should happen last after closing the exported files. So it seems that the Close() calls should be skipped on !initialized but the order of events shouldn't be changed.