syscall: add fcntl test
syscall: add fcntl test

Also updates documentation.

LGTM=minux.ma
R=iant, bradfitz, nightlyone, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/58660044

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