systemctl: Support --timestamp for otherwise named properties

systemctl: Support --timestamp for otherwise named properties

`systemctl show`'s `--timestamp` flag is supposed to reformat all
timestamp-based properties. However, the logic for detecting these
properties was incomplete and only checked if the name ended in
Timestamp.

Expand the check to explicitly include some non-"timestamp" named
properties that really are timestamps.

Fixes: https://github.com/systemd/systemd/issues/39282