luci-app-privoxy: change to tabbed display

- changed to tabbed display
- modified version detection
- adaption to new function version_compare() in ipkg.lua
- adaption to fixed Flag.parse() in cbi.lua
- add support new option "system.boot_delay"
- also fix #763

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>