LibC+userspace: Make everything compile with -Wall -Wextra -Werror

I added -Wall -Wextra -Werror as public compile flags to libc. Now
everything in userspace in compiled using these flags. I made all
necessary changes to allow compilation to work.

Only exception is execvp which has a large stack usage. Maybe it
should use malloc for the buffer but posix allows ENOMEM only when
kernel is out of memory... This can be fixed when fexecve is
implemented and there is no need for absolute path.

userspace/snake/main.cpp

@@ -27,7 +27,7 @@ bool				g_running		= true;
 Point				g_grid_size		= { 21, 21 };
 Direction			g_direction		= Direction::Up;
 Point				g_head			= { 10, 10 };
-int					g_tail_target	= 3;
+size_t				g_tail_target	= 3;
 int					g_score			= 0;
 BAN::Vector<Point>	g_tail;
 Point				g_apple;
@@ -133,6 +133,8 @@ void update()
 				if (g_direction != Direction::Left)
 					new_direction = Direction::Right;
 				break;
+			default:
+				break;
 		}
 	}
 
@@ -154,6 +156,8 @@ void update()
 		case Direction::Right:
 			g_head.x++;
 			break;
+		default:
+			ASSERT_NOT_REACHED();
 	}
 
 	if (g_head.x < 0 || g_head.y < 0 || g_head.x >= g_grid_size.x || g_head.y >= g_grid_size.y)