Add netifd support to sstp-client

Add netifd support to sstp-client, make it easy to use.

for example, add a section to /etc/config/network, establish a sstp connection to your server

    config interface 'vpn'
    	option proto 'sstp'
    	option server 'yourserver.net'
    	option username 'sstp_username'
    	option password 'sstp_password'

i also add luci interface for it, named luci-proto-sstp.