syscall : add ProcAttr field to pass an unescaped command line on windows

On windows, the command line is passed as a single null-terminated string. While the automatic parameter escaping done by syscall.StartProcess works fine with most Windows programs, some applications do their own custom parsing of the command line, in which case the automatic escaping becomes harmful.
This CL adds a new extra CmdLine field to syscall.ProcAttr that will be used as the raw/unescaped command line if not empty.
Fixes #1849.

R=golang-dev, alex.brainman, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4548050

src/pkg/syscall/exec_windows.go

@@ -14,7 +14,7 @@ import (
 
 var ForkLock sync.RWMutex
 
-// escape rewrites command line argument s as prescribed
+// EscapeArg rewrites command line argument s as prescribed
 // in http://msdn.microsoft.com/en-us/library/ms880421.
 // This function returns "" (2 double quotes) if s is empty.
 // Alternatively, these transformations are done:
@@ -23,7 +23,7 @@ var ForkLock sync.RWMutex
 // - every double quote (") is escaped by back slash (\);
 // - finally, s is wrapped with double quotes (arg -> "arg"),
 //   but only if there is space or tab inside s.
-func escape(s string) string {
+func EscapeArg(s string) string {
 	if len(s) == 0 {
 		return "\"\""
 	}
@@ -89,7 +89,7 @@ func makeCmdLine(args []string) string {
 		if s != "" {
 			s += " "
 		}
-		s += escape(v)
+		s += EscapeArg(v)
 	}
 	return s
 }
@@ -222,6 +222,7 @@ type ProcAttr struct {
 	Env        []string
 	Files      []int
 	HideWindow bool
+	CmdLine    string // used if non-empty, else the windows command line is built by escaping the arguments passed to StartProcess
 }
 
 var zeroAttributes ProcAttr
@@ -252,10 +253,19 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid, handle int,
 	}
 	argv0p := StringToUTF16Ptr(argv0)
 
+	var cmdline string
+	// Windows CreateProcess takes the command line as a single string:
+	// use attr.CmdLine if set, else build the command line by escaping
+	// and joining each argument with spaces
+	if attr.CmdLine != "" {
+		cmdline = attr.CmdLine
+	} else {
+		cmdline = makeCmdLine(argv)
+	}
+
 	var argvp *uint16
-	s := makeCmdLine(argv)
-	if len(s) != 0 {
-		argvp = StringToUTF16Ptr(s)
+	if len(cmdline) != 0 {
+		argvp = StringToUTF16Ptr(cmdline)
 	}
 
 	var dirp *uint16