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