syscall: add Dup3 on Linux

With dup3, we can avoid an extra system call on some machines
while holding syscall.ForkLock. Currently we have to
syscall.Dup + syscall.CloseOnExec.

On machines with Linux and a new enough kernel, this can just
be dup3.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12170045

src/pkg/syscall/syscall_linux.go

@@ -778,6 +778,7 @@ func Mount(source string, target string, fstype string, flags uintptr, data stri
 //sys	Creat(path string, mode uint32) (fd int, err error)
 //sysnb	Dup(oldfd int) (fd int, err error)
 //sysnb	Dup2(oldfd int, newfd int) (err error)
+//sysnb	Dup3(oldfd int, newfd int, flags 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

@@ -310,6 +310,16 @@ func Dup2(oldfd int, newfd int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Dup3(oldfd int, newfd int, flags int) (err error) {
+	_, _, e1 := RawSyscall(SYS_DUP3, uintptr(oldfd), uintptr(newfd), uintptr(flags))
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func EpollCreate(size int) (fd int, err error) {
 	r0, _, e1 := RawSyscall(SYS_EPOLL_CREATE, uintptr(size), 0, 0)
 	fd = int(r0)

src/pkg/syscall/zsyscall_linux_amd64.go

@@ -310,6 +310,16 @@ func Dup2(oldfd int, newfd int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Dup3(oldfd int, newfd int, flags int) (err error) {
+	_, _, e1 := RawSyscall(SYS_DUP3, uintptr(oldfd), uintptr(newfd), uintptr(flags))
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func EpollCreate(size int) (fd int, err error) {
 	r0, _, e1 := RawSyscall(SYS_EPOLL_CREATE, uintptr(size), 0, 0)
 	fd = int(r0)

src/pkg/syscall/zsyscall_linux_arm.go

@@ -310,6 +310,16 @@ func Dup2(oldfd int, newfd int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Dup3(oldfd int, newfd int, flags int) (err error) {
+	_, _, e1 := RawSyscall(SYS_DUP3, uintptr(oldfd), uintptr(newfd), uintptr(flags))
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func EpollCreate(size int) (fd int, err error) {
 	r0, _, e1 := RawSyscall(SYS_EPOLL_CREATE, uintptr(size), 0, 0)
 	fd = int(r0)