syscall : add a field to ProcAttr so that StartProcess can hide the executed application on windows

The SW_HIDE parameter looks like the only way for a windows GUI application to execute a CLI subcommand without having a shell windows appearing.

R=brainman, golang-dev, bradfitzgo, rsc1
CC=golang-dev
https://golang.org/cl/4439055

src/pkg/syscall/exec_windows.go

@@ -221,6 +221,7 @@ type ProcAttr struct {
 	Dir        string
 	Env        []string
 	Files      []int
+	HideWindow bool
 }
 
 var zeroAttributes ProcAttr
@@ -282,6 +283,10 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid, handle int,
 	si := new(StartupInfo)
 	si.Cb = uint32(unsafe.Sizeof(*si))
 	si.Flags = STARTF_USESTDHANDLES
+	if attr.HideWindow {
+		si.Flags |= STARTF_USESHOWWINDOW
+		si.ShowWindow = SW_HIDE
+	}
 	si.StdInput = fd[0]
 	si.StdOutput = fd[1]
 	si.StdErr = fd[2]

src/pkg/syscall/ztypes_windows_386.go

@@ -77,6 +77,7 @@ const (
 
 	HANDLE_FLAG_INHERIT    = 0x00000001
 	STARTF_USESTDHANDLES   = 0x00000100
+	STARTF_USESHOWWINDOW   = 0x00000001
 	DUPLICATE_CLOSE_SOURCE = 0x00000001
 	DUPLICATE_SAME_ACCESS  = 0x00000002
 
@@ -240,6 +241,25 @@ type ByHandleFileInformation struct {
 	FileIndexLow       uint32
 }
 
+// ShowWindow constants
+const (
+	// winuser.h
+	SW_HIDE            = 0
+	SW_NORMAL          = 1
+	SW_SHOWNORMAL      = 1
+	SW_SHOWMINIMIZED   = 2
+	SW_SHOWMAXIMIZED   = 3
+	SW_MAXIMIZE        = 3
+	SW_SHOWNOACTIVATE  = 4
+	SW_SHOW            = 5
+	SW_MINIMIZE        = 6
+	SW_SHOWMINNOACTIVE = 7
+	SW_SHOWNA          = 8
+	SW_RESTORE         = 9
+	SW_SHOWDEFAULT     = 10
+	SW_FORCEMINIMIZE   = 11
+)
+
 type StartupInfo struct {
 	Cb            uint32
 	_             *uint16