unix: change Dup,Dup2,Dup3 to use Syscall, not RawSyscall

This avoids hanging when a Go program uses a FUSE filesystem and the
dup system call has to close a file descriptor.  When dup uses
RawSyscall then the goroutine calling dup will occupy a scheduler slot
(a p structure) during the call, and may block waiting for some other
goroutine to respond to the close call on the FUSE filesystem.
Changing to Syscall avoids the problem.  This makes Dup a tiny bit
slower but is quite unlikely to make a difference for any real
programs.

Update golang/go#10202.

Change-Id: I590c5c9a04e0a1281a85dc553c7592fa83949ac7
Reviewed-on: https://go-review.googlesource.com/8056
Reviewed-by: Rob Pike <r@golang.org>

unix/zsyscall_darwin_386.go

@@ -3,7 +3,12 @@
 
 package unix
 
-import "unsafe"
+import (
+	"syscall"
+	"unsafe"
+)
+
+var _ syscall.Errno
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
@@ -409,7 +414,7 @@ func Close(fd int) (err error) {
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
 func Dup(fd int) (nfd int, err error) {
-	r0, _, e1 := RawSyscall(SYS_DUP, uintptr(fd), 0, 0)
+	r0, _, e1 := Syscall(SYS_DUP, uintptr(fd), 0, 0)
 	nfd = int(r0)
 	if e1 != 0 {
 		err = e1
@@ -420,7 +425,7 @@ func Dup(fd int) (nfd int, err error) {
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
 func Dup2(from int, to int) (err error) {
-	_, _, e1 := RawSyscall(SYS_DUP2, uintptr(from), uintptr(to), 0)
+	_, _, e1 := Syscall(SYS_DUP2, uintptr(from), uintptr(to), 0)
 	if e1 != 0 {
 		err = e1
 	}