siproxd: use UCI callback processing, reduce code size

siproxd: use UCI callback processing, reduce code size

Switch to using callback functions for UCI config processing, and remove
the unnecessary scan_interfaces() call together with related code.

Instead of explicitly handling every possible siproxd config option, use
callbacks to deal with those defined in the UCI config file, and directly
handle only selected options with desired default values. This shrinks
the init code by ~3.5 KB and simplifies updating to future siproxd
versions with new options.

(Note: this change exposed some long-standing, broken aspects of UCI
callback handling, fixed in https://github.com/openwrt/openwrt/pull/805.)

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>