kern: tty: refactor TIOCSTI privilege checks slightly

[kevans]

Summary

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.

[markj]

sys/kern/tty.c

1653

Or just if (priv_check(td, PRIV_TTY_STI) == 0) return (0);.

[kib]

sys/kern/tty.c

1656

This is counter-intuitive BTW.

[kevans]

sys/kern/tty.c

1656

I agree, but I haven't yet spent any time tracking down the current requirements -- I would've expected to need FWRITE instead to alter the tty buffer.