luci-app-statistics: Adjust UI defaults to match config file

luci-app-statistics: Adjust UI defaults to match config file

Adjust the defaults shown in the LuCI user interface to match
the real default values in the default config file.

(If a plugin is disabled and config values get deleted from
the config file, user has been offered incorrect default
values from UI defaults when the plugin is later re-enabled.)

* email: socket in /var/run/collectd/ dir
* ping: TTL 127
* rrdtool: 3600 sec (1hour) as shortest period, 144 RRArows
* unixsock: socket in /var/run/collectd/ dir

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
(Backported and adapted from commit f7e5b56649)