Remove unneded check. No need to do m_pullup to the size that we prepended.

Sponsored by:	Yandex LLC
Remove unneded check. No need to do m_pullup to the size that we prepended.

MFC after:	1 week
Sponsored by:	Yandex LLC
Rewrite parts of gpioctl(8) to use the gpio(3) library.
Fix an off-by-one in gpio_pin_list().

Coverity CID:	1256495
Fix null pointer dereference.

MFC after:	2 weeks
Coverity CID:	1256497
Sponsored by:	The FreeBSD Foundation