archive/tar: simplify use of constants in test case.

Replace setsid with c_ISGID since the constant is already defined.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7403048

Committer: Brad Fitzpatrick <bradfitz@golang.org>
cmd/vet: check argument types in printf formats
Fixes issue 4404.

R=gri, rsc
CC=golang-dev
https://codereview.appspot.com/7378061
syscall: add if_announce support for freebsd

Update issue 4866.

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7398047
syscall: add if_announce support for netbsd

Update issue 4866.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/7382052
syscall: add if_announce support for openbsd

Update issue 4866.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7382053
syscall: add network interface announce support on BSD variants

This CL allows to receive network interface arrival and depature
notifications through routing sockets on BSD variants. So far
Darwin doesn't support this feature.

Also does small simplification.

Update issue 4866.

R=golang-dev, lucio.dere, dave
CC=golang-dev
https://codereview.appspot.com/7365055