added SHUT_RD, SHOT_WR and xSHUT_RDWR to fix net compilation error
added SHUT_RD, SHOT_WR and xSHUT_RDWR to fix net compilation error

R=rsc
http://codereview.appspot.com/165044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go