linprocfs: Make sure to report -1 as tty when we have no controlling tty.

When reporting a process' stats, we can't just provide the tty as an
unsigned long, as if we have no controlling tty, the tty would be NODEV, or
-1. Instaed, just special-case NODEV.

Submitted by: Juraj Lutter <otis@sk.FreeBSD.org>
MFC after: 1 week