This allows cleanly exiting a console session without control sequences.

Relates to https://github.com/lxc/lxd/pull/4001 .

Note that the existence of a signal handler now doesn't guarantee that ts->node
is allocated. Instead, ts->node will now only be added to if stdinfd is a tty.
New checks need to take that into account.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>