subprocess: Do not escape double quotes for command line arguments on Windows

* refactor: Guard `util::quote_argument()` with `#ifdef __USING_WINDOWS__`

The `util::quote_argument()` function is specific to Windows and is used
in code already guarded by `#ifdef __USING_WINDOWS__`.

* Do not escape double quotes for command line arguments on Windows

This change fixes the handling of double quotes and aligns the behavior
with Python's `Popen` class. For example:
```
>py -3
>>> import subprocess
>>> p = subprocess.Popen("cmd.exe /c dir \"C:\\Program Files\"", stdout=subprocess.PIPE, text=True)
>>> print(f"Captured stdout:\n{stdout}")
```

Currently, the same command line processed by the `quote_argument()`
function looks like `cmd.exe /c dir "\"C:\Program" "Files\""`, which is
broken.

With this change, it looks correct: `cmd.exe /c dir "C:\Program Files"`.

Github-Pull: arun11299/cpp-subprocess#113
Rebased-From: ed313971c04ac10dc006104aba07d016ffc6542a

Author: hebasto

src/util/subprocess.h

@@ -171,6 +171,7 @@ class OSError: public std::runtime_error
 //--------------------------------------------------------------------
 namespace util
 {
+#ifdef __USING_WINDOWS__
   inline void quote_argument(const std::wstring &argument, std::wstring &command_line,
                       bool force)
   {
@@ -181,7 +182,7 @@ namespace util
     //
 
     if (force == false && argument.empty() == false &&
-        argument.find_first_of(L" \t\n\v\"") == argument.npos) {
+        argument.find_first_of(L" \t\n\v") == argument.npos) {
       command_line.append(argument);
     }
     else {
@@ -231,7 +232,6 @@ namespace util
     }
   }
 
-#ifdef __USING_WINDOWS__
   inline std::string get_last_error(DWORD errorMessageID)
   {
     if (errorMessageID == 0)