xwayland: Refactor argument string construction

Replace an oft-duplicated pattern with a trivial helper function. In
doing so, we observe that the one special case (displayfd 'didn't need
to be CLOEXEC') was wrong, because the X server does fork itself
internally, so there is nothing wrong with setting CLOEXEC.

Signed-off-by: Daniel Stone <daniels@collabora.com>