Implement automatic adaptation for the codel latency target in functions.sh

The helper function get_target() now checks and sanitizes the inputs
to follow the demands of tc-codel, that is number followed by unit of time,
with s, sec, secs, ms, msec, msecs, us, usec, usecs all being valid.
Alternatively the function will calculate an increase of target for
link speeds below roughly 2500kbps if the user specified "auto" in
the GUI.
Switch simple and simplest.qos to the new get_target helper

The qdisc setup scripts now need to pass the respective link speed
for up- and download. Also the target paramter needs to be "quoted"
to allow spaces in the input fields, so that "10 ms" becomes valid.
Mention the latency auto adjustment in the GUI

NT