Add a new ioctl to allow the setting of GPIO pin names.

When a gpiobus child is added, use its name to identify the mapped pin
names.

Make the respective changes to libgpio.

Add a new '-n' flag to gpioctl(8) to set the pin name.

Differential Revision: https://reviews.freebsd.org/D2002
Reviewed by: rpaulo
Requested by: many