syscalls: make use of stderr and fix chdir syscall return value

src/drivers/screen.c

@@ -36,6 +36,11 @@ void terminal_setcolor(uint8_t color)
         terminal_color = color;
 }
 
+uint8_t terminal_getcolor()
+{
+        return terminal_color;
+}
+
 void terminal_putentryat(char c, uint8_t color, size_t x, size_t y) 
 {
         const size_t index = y * VGA_WIDTH + x;