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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang