pty: set argv via helper function

pty: set argv via helper function

This makes the pty layer independent of kmscon global state. This allows
us to use it in other applications bundled with kmscon. There is still
some work to do to make it fully independent so we can integrate it into
TSM. But that's not really needed, yet.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>