shadowsocks-libev: validate bool with default value 0

shadowsocks-libev: validate bool with default value 0

The change is mainly for keeping compatibility with old validate_data
before ubox commit ac481cdd999ee84d3f115c33a56397237e95ec64 in Sat Jul
16 14:52:36 2016 +0200.  The behaviour change comes with that commit can
be seen with the following command line session

    root@LEDE:/usr/bin# validate_data network interface wan 'disabled:bool:false'
    network.wan.disabled is unset and defaults to bool false
    disabled=0; root@LEDE:/usr/bin#

    root@OpenWrt:/# validate_data network interface lan 'disabled:bool:false'
    disabled='false'; root@OpenWrt:/#

This will cause shadowsocks-libev in current master branch fail on OpenWrt
15.01 though they actually should only use packages from the 15.01 branch...

Fixes openwrt/packages#4614

Signed-off-by: Yousong Zhou <redacted>