Make linprocfs(5) report correct tty number in /proc/<PID>/stat.
Fixes sudo (sudo-1.8.21p2-3ubuntu1.2); previously would fail
with "sudo: no tty present and no askpass program specified".

Reviewed by: kib, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25588