gpioctl - manage GPIO pins under freebsd

• simple go library to manage GPIO pins under freebsd
• it's uses the 'ioctl()' system call to manage the gpio pins.
• tested on 'FreeBSD rpi2 11.0-CURRENT FreeBSD 11.0-CURRENT #0 r296485'
  • doesn't work under FreeBSD 10 - missing ioctl to name a pin (GPIOSETNAME)
  • 'GPIOSETNAME' was implemented in r279761 (git sha: 04b6b93)
• no interrupts - currently not supported under freebsd
• check the 'examples' directory for examples