linux: Forbid legacy TIOCSTI usage

linux: Forbid legacy TIOCSTI usage

To quote from the kernel documentation:

> Historically the kernel has allowed TIOCSTI, which will push
> characters into a controlling TTY. This continues to be used
> as a malicious privilege escalation mechanism, and provides no
> meaningful real-world utility any more. Its use is considered
> a dangerous legacy operation, and can be disabled on most
> systems.
>
> Say Y here only if you have confirmed that your system's
> userspace depends on this functionality to continue operating
> normally.
>
> Processes which run with CAP_SYS_ADMIN, such as BRLTTY, can
> use TIOCSTI even when this is set to N.
>
> This functionality can be changed at runtime with the
> dev.tty.legacy_tiocsti sysctl. This configuration option sets
> the default value of the sysctl.

This patch therefore proposes to no longer allow legacy TIOCSTI usage
in IPFire, given its security implications and the apparent lack of
legitimate usage.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>