rc.d/motd: Update motd more robustly

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

Reported by: Jonathan Walton <jonathan AT isilon.com>
Reviewed by: allanjude, vangyzen
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20701