[SQM/luci-app-sqm] Wire up link layer dandling for cake

[SQM/luci-app-sqm] Wire up link layer dandling for cake

The cake traffic-shaper qdisc omne stop solution knows how to handle
link layer adjustments for ATM and can account for per packet overhead.
This commit adds cake as link layer adjustment mechanism in the GUI and
passes numerically specified overhead as well as the ATM linklayer
keywords on to cake. This change also passes the "advanced option strings"
from the Queue Discipline tab to cake. But as before no error checking.
This needs testing, as I have no working cake qdisc available so
caveat emptor...

Signed-off-by: Sebastian Moeller <redacted>