syscall: make use of include/linux when generating system constants
syscall: make use of include/linux when generating system constants

On Linux include/net directory is just to help porting applications
from BSDs and files under net keep less information than include/linux.
Making use of files under include/linux instead of include/net prevents
lack of information.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/63930043

rdp/rogerpack2005-golang,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh