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.)

* dns: set br-lan as the interface
* email: socket in /var/run/collectd/ dir
* interface: set br-lan as the interface
* ping: TTL 127, interval 30
* rrdtool: 1hour as shortest period, 144 RRArows
* tcpconns: Do not monitor all, only 22 80
* unixsock: socket in /var/run/collectd/ dir

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>