Rev. 1.274 put the ttyrel() call before the destroy_dev() in the

Rev. 1.274 put the ttyrel() call before the destroy_dev() in the
ttyfree(), freeing the tty. Since destroy_dev() may call d_purge()
cdevsw method, that is the ttypurge() for the tty, the code ends up
accessing freed tty structure.

Put the ttyrel() after destroy_dev() in the ttyfree. To prevent the
panic the rev. 1.274 provided fix for, check the TS_GONE in sysctl
handler and refuse to provide information on such tty.

Reported, debugging help and tested by:	pho
DIscussed with and reviewed by:	jhb
MFC after:	1 week