Add a backup/restore capability for rrd data storage in
luci_statistics. The data storage is typically in /tmp and does not
survive reboot or sysupgrade. This adds an option for the
administrator to configure the RRD plugin, so that the RRD data are
are preserved with a backup copy in the overlay file system.
This works for shutdown/reboot, sysupgrade (backup config files,
restore config files, and true sysupgrade).
Also fix a bug where starting luci_statistics for the first time would
not get a restart a running collectd: during install of the package
when it is not included in the base flashed image, collectd might be
started when it got installed/configured before this package gets
installed/configured. So we need to check if it's running, and
restart it to use the luci_statistics configuration.
Signed-off-by: John Kohl <jtk.git@bostonpog.org>
ad98af3a2b