rc.d/motd: Update motd more robustly

[cem]

Summary

Use appropriate fsyncs to persist the rewritten /etc/motd file, when a
rewrite is performed.

[cem]

As mjg@ points out, the mktemp file may be on a different filesystem than /etc
(e.g., tmpfs, but any other filesystem invalidates the initial fsync). Copy to
a temporary /etc file, sync that, then rename over /etc/motd.