Add netifd script to control sstp connections.

Sorry , my bad,  my poor English. 
But I am not modify 'netifd' at all. 
Only add one script to controlling SSTP connections, and a little modify of Makefile.

This is my first time use 'git'. Sorry for last pull request, most of them are valid.

So, only changes of 'sstp-client' package. Should i patch here or mail to openwrt.org?