net: add missing SetKeepAlivePeriod for Plan 9
net: add missing SetKeepAlivePeriod for Plan 9

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/12082045

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