lib/oe/terminal: Fix tmux new-session on older tmux versions (<1.9)

`tmux new -c` fails on tmux older than 1.9, when that flag was added.
We can omit the flag for older versions of tmux, and the working
directory gets set even without it.

Signed-off-by: Peter Budny <pbbudny@amazon.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c55c294b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>