syscall: make linux Dup2 match other systems

You could argue for changing all the others, but
Linux is outvoted, and the only time it matters
is when newfd==-1, in which case you can call Dup.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5650073

src/pkg/syscall/syscall_linux.go

@@ -802,7 +802,7 @@ func Mount(source string, target string, fstype string, flags uintptr, data stri
 //sys	Close(fd int) (err error)
 //sys	Creat(path string, mode uint32) (fd int, err error)
 //sysnb	Dup(oldfd int) (fd int, err error)
-//sysnb	Dup2(oldfd int, newfd int) (fd int, err error)
+//sysnb	Dup2(oldfd int, newfd int) (err error)
 //sysnb	EpollCreate(size int) (fd int, err error)
 //sysnb	EpollCreate1(flag int) (fd int, err error)
 //sysnb	EpollCtl(epfd int, op int, fd int, event *EpollEvent) (err error)

src/pkg/syscall/zsyscall_linux_386.go

@@ -210,9 +210,8 @@ func Dup(oldfd int) (fd int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Dup2(oldfd int, newfd int) (fd int, err error) {
-	r0, _, e1 := RawSyscall(SYS_DUP2, uintptr(oldfd), uintptr(newfd), 0)
-	fd = int(r0)
+func Dup2(oldfd int, newfd int) (err error) {
+	_, _, e1 := RawSyscall(SYS_DUP2, uintptr(oldfd), uintptr(newfd), 0)
 	if e1 != 0 {
 		err = e1
 	}

src/pkg/syscall/zsyscall_linux_amd64.go

@@ -210,9 +210,8 @@ func Dup(oldfd int) (fd int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Dup2(oldfd int, newfd int) (fd int, err error) {
-	r0, _, e1 := RawSyscall(SYS_DUP2, uintptr(oldfd), uintptr(newfd), 0)
-	fd = int(r0)
+func Dup2(oldfd int, newfd int) (err error) {
+	_, _, e1 := RawSyscall(SYS_DUP2, uintptr(oldfd), uintptr(newfd), 0)
 	if e1 != 0 {
 		err = e1
 	}

src/pkg/syscall/zsyscall_linux_arm.go

@@ -210,9 +210,8 @@ func Dup(oldfd int) (fd int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Dup2(oldfd int, newfd int) (fd int, err error) {
-	r0, _, e1 := RawSyscall(SYS_DUP2, uintptr(oldfd), uintptr(newfd), 0)
-	fd = int(r0)
+func Dup2(oldfd int, newfd int) (err error) {
+	_, _, e1 := RawSyscall(SYS_DUP2, uintptr(oldfd), uintptr(newfd), 0)
 	if e1 != 0 {
 		err = e1
 	}