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

include/screen.h

@@ -34,6 +34,7 @@ uint16_t vga_entry(unsigned char uc, uint8_t color);
 size_t strlen(const char* str);
 void terminal_initialize(void);
 void terminal_setcolor(uint8_t color);
+uint8_t terminal_getcolor();
 void terminal_putentryat(char c, uint8_t color, size_t x, size_t y);
 void terminal_putchar(char c);
 void terminal_write(const char* data, size_t size);