Import libgpio.

[rpaulo]

Summary

libgpio is a simple library to deal with the GPIO kernel interface.
Right now, it's pretty basic, but we can improve it in the future.

[loos]

Are we going to change gpioctl(1) to use libgpio ?

Thanks Rui!

[rpaulo]

In D1183#5, @loos wrote:

Are we going to change gpioctl(1) to use libgpio ?

Thanks Rui!

Yes, in a separate commit.

[loos]

Absolutely!

[rpaulo]

Closed by commit rS274987 (authored by @rpaulo).