Add netifd script to control sstp connections.

Sorry for last pull request, most of them are valid.It's maybe my first time use 'git'.

I only add one script to controlling SSTP connections, and a little modify of Makefile. So you can add a new sstp connection in '/etc/config/network' like this:

    config interface 'vpn'
        option proto 'sstp'
        option server 'yourserver.net'
        option username 'sstp_username'
        option password 'sstp_password'
luci example:(luci support package have pull request to https://github.com/openwrt/luci)
<https://www.dropbox.com/s/n1wsem74pjcjhyu/luci-proto-sstp.png?dl=0>