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/syscall_linux.go

@@ -591,7 +591,7 @@ func PtraceDetach(pid int) (errno int) { return ptrace(PTRACE_DETACH, pid, 0, 0)
 //sys	Sethostname(p []byte) (errno int)
 //sys	Setpgid(pid int, pgid int) (errno int)
 //sys	Setrlimit(resource int, rlim *Rlimit) (errno int)
-//sys	Setsid() (pid int)
+//sys	Setsid() (pid int, errno int)
 //sys	Settimeofday(tv *Timeval) (errno int)
 //sys	Setuid(uid int) (errno int)
 //sys	Splice(rfd int, roff *int64, wfd int, woff *int64, len int, flags int) (n int64, errno int)