Convert to LIBADD.

Reviewed by:	bapt
Import libgpio.

This is a thin wrapper around the kernel interface which should make
it easier to write GPIO applications.  gpioctl(8) will be converted to
use this library in a separate commit.

Differential Revision:	https://reviews.freebsd.org/D1183
Reviewed by:	adrian, loos
Discussed on:	arm@, embedded@
Relnotes:	yes
Skip L2 addresses lookups for p2p interfaces.

Discussed with:	melifaro
Sponsored by:	Yandex LLC