Remove duplicated rate fields

Remove one copy of the accidentally duplicated up- and download
input fields.
Introduce Advanced checkbox to simplify default GUI

The advanced checkbox will allow to hide the actual Queueing discipline
and Queue setup script drop-down-lists to not irritate users too much.
Teach run.sh to set default qdisc and qos-script

Since hidden fields in the LuCI GUI do not seem accessible from scripts,
run.sh will have to select default values for the qdisc and the script that
sets up the actual latency control machinery. This is suboptimal, since now
run.sh also contains policy (namely default script and qdisc). This might
need revisting in the future...
Hide qdisc and qos-script lists unless advanced configuration is requested

Only display the Queueing discipline and Queue setup script drop down lists
if the user checked the Advanced Configuration check box. The goal is to
simplify the GUI without taking the configurability away.
Hide linklayer adaptation parameters unless a linklayer is selected

Only show the linklayer adapaptation parameters, if the user selected
a linklayer to adapt for other than none. The goal is to keep the GUI
simple but powerful.
Allow up- & download-rates >= 0

Allow rates of 0 kbit/s to allow selectively disable bandwidth shaping
independently for uplink and downlink.
Improve the descriptions of the linklayer adaptation parameters

The linklayer adaptation parameters are confusing, so improve the descriptions
to keep the user on-trak.