create /var/run/motd if missing when update_motd="NO"
There are three cases where /var/run/motd won't exist if update_motd="NO"
- /var/run is tmpfs.
- doing first install and setting update_motd="NO" before rebooting.
- upgrading from a version that used /etc/motd.
Copy /etc/motd if it is readable to preserve motd if upgrading
from a version before /var/run/motd was introduced.
Otherwise, copy /etc/motd.template.