os: use `return none` in functions returning `?`

vlib/os/os.v

@@ -196,7 +196,7 @@ pub fn cp(old, new string) ? {
 			return error_with_code('failed to set permissions for $new', int(-1))
 		}
 	}
-	return
+	return none
 }
 
 [deprecated]
@@ -225,7 +225,7 @@ pub fn cp_all(osource_path, odest_path string, overwrite bool) ? {
 		os.cp(source_path, adjusted_path) or {
 			return error(err)
 		}
-		return
+		return none
 	}
 	if !os.is_dir(dest_path) {
 		return error('Destination path is not a valid directory')
@@ -246,7 +246,7 @@ pub fn cp_all(osource_path, odest_path string, overwrite bool) ? {
 			panic(err)
 		}
 	}
-	return
+	return none
 }
 
 // mv_by_cp first copies the source file, and if it is copied successfully, deletes the source file.
@@ -258,7 +258,7 @@ pub fn mv_by_cp(source string, target string) ? {
 	os.rm(source) or {
 		return error(err)
 	}
-	return
+	return none
 }
 
 // vfopen returns an opened C file, given its path and open mode.
@@ -638,23 +638,24 @@ pub fn rm(path string) ? {
 			return error(posix_get_error_msg(C.errno))
 		}
 	}
-
-	return
+	return none
 	// C.unlink(path.cstr())
 }
 // rmdir removes a specified directory.
 pub fn rmdir(path string) ? {
-	$if !windows {
+	$if windows {
+		rc := C.RemoveDirectory(path.to_wide())
+		if rc == 0 {
+			// https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-removedirectorya - 0 is failure
+			return error('Failed to remove "$path"')
+		}
+	} $else {
 		rc := C.rmdir(path.str)
 		if rc == -1 {
 			return error(posix_get_error_msg(C.errno))
 		}
-	} $else {
-		rc := C.RemoveDirectory(path.to_wide())
-		if rc == 0 {
-			return error('Failed to remove "$path"')
-		}
 	}
+	return none
 }
 
 [deprecated]
@@ -666,7 +667,7 @@ pub fn rmdir_recursive(path string) {
 pub fn rmdir_all(path string) ? {
 	mut ret_err := ''
 	items := os.ls(path) or {
-		return
+		return none
 	}
 	for item in items {
 		if os.is_dir(os.join_path(path, item)) {
@@ -678,6 +679,7 @@ pub fn rmdir_all(path string) ? {
 	if ret_err.len > 0 {
 		return error(ret_err)
 	}
+	return none
 }
 
 pub fn is_dir_empty(path string) bool {