Write and use create_protected_file() helper

Instead of using umask to ensure sensitive files are created with
restrictive permissions, git-crypt now does:

	create_protected_file(filename);
	std::ofstream out(filename);
	// ...

create_protected_file can have different Unix and Windows implementations.
create_protected_file should be easier to implement on Windows than a
umask equivalent, and this pattern keeps the amount of platform-specific
code to a minimum and avoids #ifdefs.

util-win32.cpp

@@ -346,10 +346,8 @@ static void	init_std_streams_platform ()
 	_setmode(_fileno(stdout), _O_BINARY);
 }
 
-mode_t util_umask (mode_t mode)
+void create_protected_file (const char* path) // TODO
 {
-	// Not available in Windows and function not always defined in Win32 environments
-	return 0;
 }
 
 int util_rename (const char* from, const char* to)