bus: rename message "serial" to "cookie"

Even if the lower-leveld dbus1 protocol calls it "serial", let's expose
the word "cookie" for this instead, as this is what kdbus uses and since
it doesn't imply monotonicity the same way "serial" does.
man: document sd_bus_message_get_cookie()
man: add missing sd_bus_message_get_cookie man page fie