bus: conditionally set cookie_reply/timeout and update kdbus.h
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
bus: add a bit of pure/const decorators
journalctl: remove unexpected behavior of journalctl -b

This flag shouldn't try and consume the following argument. It should
behave like every other flag which takes an optional argument when
parsed by getopt_long.
completion: don't offer completions for journalctl -b

Since this is a shortopt with an optional argument, assume the user
knows what they're doing. The longopts --boot and --this-boot will
continue to offer boot IDs as completions.