kern: tty: refactor TIOCSTI privilege checks slightly

This removes some repetition from it and makes the flow a little more
obvious. Future work may find some way to add more constraints to the
unprivileged path, add a security sysctl to disable it, or perhaps
some combination of the two.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D50506