check if TrafficControl is enabled before testing imq module requirement.

This avoids failing to start in current OpenWrt Attitude Adjustment and trunk,
which lack IMQ support (and WONTFIX). Suggested in the issue discussed in:
http://ml.ninux.org/pipermail/nodogsplash/2013-May/000009.html

Signed-off-by: Gui Iribarren <gui@altermundi.net>
recognize yes, true or 1 as valid TrafficControl values

Signed-off-by: Gui Iribarren <gui@altermundi.net>
fix compiler warnings
disable all traffic control code when traffic_control is set to 0