Linux: fix RA time unit confusion

Linux: fix RA time unit confusion

The RA times are provided in milliseconds, but commit
569051c8aa8fc297eb8edb7bd228e0fd353d30c1 (dhcpcd-8.1.3~18) “Linux:
prefer ms RA times” incorrectly scaled them by an extra factor of 1000
before writing them to retrans_time_ms and base_reachable_time_ms.
Instead, write the values in milliseconds directly to the *_ms files,
and correctly convert to jiffies or seconds for the other files if
necessary.

Fixes NixOS/nixpkgs#76710.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>