coredumpctl: show --help text if "coredumpctl help" is called
    
    Most of our programs that take "verbs" make the "help" verb either
    equivalent to passing the --help switch (or at least print a message
    redirecting the user to that switch). Do so in coredumpctl too, in order
    to minimize surprises.
    
    (cherry picked from commit 6d8be376e1682a79f0aecceb2136884c5b4327e2)