Use TAILQ instead of STAILQ for kqeueue filedescriptors to ensure constant
time removal on kqueue close.

Reported and tested by:	pho
Reviewed by:	jmg
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (delphij)
Add gpio(4) man page to attempt to document the current hints based setup of
pin outputs, functions and setup.

Add cross reference in gpioctl(8) for people to find.

This is by no means complete and really only covers gpioled(4) and the
Atheros based systems who expose a few extra hints at boot time.

This should be updated by developers who know more about this system than
I and viewed as the beginning of documentation, not the end.

Reviewed by:	adrian
Approved by:	re (joel)
MFC after:	2 weeks