Windows:Have native CommandLine in Process

Signed-off-by: John Howard <jhoward@microsoft.com>

This adds a new field `CommandLine` in the `Process` structure for use
on Windows. A Windows runtime will check this first and use it as-is when
launching WCOW processes in a container. If omitted, the Windows runtime
will fall back to the previous behaviour of escaping (eg Windows.EscapeArg
in golang) each of the `args` array elements, and space-concatenating them.

The reason for this change is to avoid loss of fidelity for launching
processes. One such example is the following:

`cmd /S /C mkdir "c:/foo"`

When parsed into a JSON array such as `Args`, it becomes 5 elements

 - cmd
 - /S
 - /C
 - mkdir
 - c:/foo

Here, note the lost information being the double-quotes around `c:/foo`.
When using the required contenation, space separation required on Windows,
(https://github.com/golang/sys/blob/c4afb3effaa53fd9a06ca61262dc7ce8df4c081b/windows/exec_windows.go#L9-L18),
this becomes the following command line:

`cmd /S /C mkdir c:/foo`

When the double-quotes are missing, mkdir would fail, but with the
double-quotes, it succeeds as expected:

```
C:\>cmd /s /c mkdir c:/foo
The syntax of the command is incorrect.

C:\>cmd /s /c mkdir "c:/foo"

C:\>
```

The addition of a full `commandLine` in Process for use on Windows alleviates
issues where fidelity can be lost.

Some further background:

For historical reasons, Windows only has native support for launching
processes using a command line. It does not support an argument array as
per Linux. See the `CreateProcess` API documentation on MSDN.

What happens under the covers is that prior to invoking a programs
main, the language runtime will convert the command line to a set of argv[]
suach as in the C-style `int main(int argc, char* argv), or the golang
`os.Args` prior to the programs `main` being invoked, using Windows-
specific rules.

In go, that's the `commandLineToArgv` function which is called in the init()
function of the os package https://github.com/golang/go/blob/ff7b245a31394b700a252fd547cf16ad0ad838b6/src/os/exec_windows.go#L100,

In the Microsoft C++ startup code, it does exactly the same processing
as documented here prior to invoking main itself:
https://docs.microsoft.com/en-us/cpp/cpp/parsing-cpp-command-line-arguments?view=vs-2017.
The processing it describes is documented at
https://msdn.microsoft.com/en-us/library/windows/desktop/ms683156(v=vs.85).aspx and
https://docs.microsoft.com/en-us/windows/desktop/api/shellapi/nf-shellapi-commandlinetoargvw

Some related links which provide a lot more information about the
very specific (and unique to Windows) command line escaping rules,
and handling of them are below:

https://blogs.msdn.microsoft.com/twistylittlepassagesallalike/2011/04/23/everyone-quotes-command-line-arguments-the-wrong-way/
https://stackoverflow.com/questions/31838469/how-do-i-convert-argv-to-lpcommandline-parameter-of-createprocess
https://docs.microsoft.com/en-us/cpp/cpp/parsing-cpp-command-line-arguments?view=vs-2017

Author: John Howard

config.md

@@ -155,8 +155,11 @@ For POSIX platforms the `mounts` structure has the following fields:
 * **`cwd`** (string, REQUIRED) is the working directory that will be set for the executable.
     This value MUST be an absolute path.
 * **`env`** (array of strings, OPTIONAL) with the same semantics as [IEEE Std 1003.1-2008's `environ`][ieee-1003.1-2008-xbd-c8.1].
-* **`args`** (array of strings, REQUIRED) with similar semantics to [IEEE Std 1003.1-2008 `execvp`'s *argv*][ieee-1003.1-2008-functions-exec].
-    This specification extends the IEEE standard in that at least one entry is REQUIRED, and that entry is used with the same semantics as `execvp`'s *file*.
+* **`args`** (array of strings, OPTIONAL) with similar semantics to [IEEE Std 1003.1-2008 `execvp`'s *argv*][ieee-1003.1-2008-functions-exec].
+    This specification extends the IEEE standard in that at least one entry is REQUIRED (non-Windows), and that entry is used with the same semantics as `execvp`'s *file*. This field is OPTIONAL on Windows, and `commandLine` is REQUIRED if this field is omitted.
+* **`commandLine`** (string, OPTIONAL) specifies the full command line to be executed on Windows.
+    This is the preferred means of supplying the command line on Windows. If omitted, the runtime will fall back to escaping and concatenating fields from `args` before making the system call into Windows.
+
 
 ### <a name="configPOSIXProcess" />POSIX process
 

schema/config-schema.json

@@ -50,13 +50,15 @@
         "process": {
             "type": "object",
             "required": [
-                "cwd",
-                "args"
+                "cwd"
             ],
             "properties": {
                 "args": {
                     "$ref": "defs.json#/definitions/ArrayOfStrings"
                 },
+                "commandLine": {
+                    "type": "string"
+                },
                 "consoleSize": {
                     "type": "object",
                     "required": [

specs-go/config.go

@@ -38,7 +38,9 @@ type Process struct {
 	// User specifies user information for the process.
 	User User `json:"user"`
 	// Args specifies the binary and arguments for the application to execute.
-	Args []string `json:"args"`
+	Args []string `json:"args,omitempty"`
+	// CommandLine specifies the full command line for the application to execute on Windows.
+	CommandLine string `json:"commandLine,omitempty" platform:"windows"`
 	// Env populates the process environment for the process.
 	Env []string `json:"env,omitempty"`
 	// Cwd is the current working directory for the process and must be