Fixes Debian bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=921820
Introduce a new --log option to fwupdmgr that will log stdout to an argument.
If run under systemd, prefix that argument with $RUNTIME_DIRECTORY.
Add a new systemd unit and associated timer to regularly refresh metadata.
After the metadata refresh is complete, save the output to the motd location.
The timer and service are disabled by default and can be enabled by an admin.
