systemctl: explicitly cast the constants to uint64_t

systemctl: explicitly cast the constants to uint64_t

Otherwise under certain conditions `va_arg()` might get garbage instead
of the expected value, i.e.:

$ sudo build-o0/systemctl disable asdfasfaf
sd_bus_message_appendv: Got uint64_t: 0
Failed to disable unit: Unit file asdfasfaf.service does not exist.

$ sudo build-o1/systemctl disable asdfasfaf
sd_bus_message_appendv: Got uint64_t: 7954875719681572864
Failed to disable unit: Invalid argument

(reproduced on an armv7hl machine)

Resolves: #26568
Follow-up to: bf1bea43f15
Related issue: https://github.com/systemd/systemd/pull/14470#discussion_r362893735