manager: restrict Dump*() to privileged callers or ratelimit

manager: restrict Dump*() to privileged callers or ratelimit

Dump*() methods can take quite some time due to the amount of data to
serialize, so they can potentially stall the manager. Make them
privileged, as they are debugging tools anyway. Use a new 'dump'
capability for polkit, and the 'reload' capability for SELinux, as
that's also non-destructive but slow.

If the caller is not privileged, allow it but rate limited to 10 calls
every 10 minutes.