syscall: match linux Setsid function signature to darwin

SETSID does return an errno - any reason why it has been done this
way in zsyscall_linux_* ? Otherwise it should be the same as darwin.

From SETSID(2) on my Linux box:

ERRORS
       On error, -1 is returned, and errno is set.

Fixes #730

R=rsc
CC=golang-dev
https://golang.org/cl/878047

src/pkg/syscall/zsyscall_linux_arm.go

@@ -434,9 +434,10 @@ func Setrlimit(resource int, rlim *Rlimit) (errno int) {
 	return
 }
 
-func Setsid() (pid int) {
-	r0, _, _ := Syscall(SYS_SETSID, 0, 0, 0)
+func Setsid() (pid int, errno int) {
+	r0, _, e1 := Syscall(SYS_SETSID, 0, 0, 0)
 	pid = int(r0)
+	errno = int(e1)
 	return
 }