terminal-util: when querying bg color, ensure input fd and output fd refer to same tty

terminal-util: when querying bg color, ensure input fd and output fd refer to same tty

Let's add an extra safety check: before issuing the ansi sequence to
query the bg color, let's make sure input and output fd actually
reference the same tty. because otherwise it's unlikely we'll be able to
read back the response from the tty driver.

This is mostly just paranoia.