various: don't say that the timestamp 'changed' on initial load

various: don't say that the timestamp 'changed' on initial load

I always found this a bit annoying.
With the patch:

$ SYSTEMD_LOG_LEVEL=debug build/udevadm test /sys/class/block/dm-1
...
Loaded timestamp for '/etc/systemd/network'.
Loaded timestamp for '/usr/lib/systemd/network'.
Parsed configuration file /usr/lib/systemd/network/99-default.link
Parsed configuration file /etc/systemd/network/10-eth0.link
Created link configuration context.
Loaded timestamp for '/etc/udev/rules.d'.
Loaded timestamp for '/usr/lib/udev/rules.d'.
...