fs/Traits: enable _UNICODE on Windows

Use wchar_t for everything on Windows.  Solves a lot of filesystem
charset problems.

src/CommandLine.cxx

@@ -35,6 +35,7 @@
 #include "fs/Traits.hxx"
 #include "fs/FileSystem.hxx"
 #include "fs/StandardDirectory.hxx"
+#include "util/Macros.hxx"
 #include "util/Error.hxx"
 #include "util/Domain.hxx"
 #include "util/OptionDef.hxx"
@@ -337,7 +338,19 @@ parse_cmdline(int argc, char **argv, struct options *options,
 
 	if (config_file != nullptr) {
 		/* use specified configuration file */
+#ifdef _UNICODE
+		wchar_t buffer[MAX_PATH];
+		auto result = MultiByteToWideChar(CP_ACP, 0, config_file, -1,
+						  buffer, ARRAY_SIZE(buffer));
+		if (result <= 0) {
+			error.SetLastError("MultiByteToWideChar() failed");
+			return false;
+		}
+
+		return ReadConfigFile(Path::FromFS(buffer), error);
+#else
 		return ReadConfigFile(Path::FromFS(config_file), error);
+#endif
 	}
 
 	/* use default configuration file path */