XUngrabKeyboard() instead of XUngrabKey()

XUngrabKeyboard() instead of XUngrabKey()

XUngrabKey(), which is currently used in cleanup(), is not the right
counterpart to XGrabKeyboard(), which is used in grabkeyboard(),
called from main().

XUngrabKeyboard() is the function to use, as grabbing the whole
keyboard is different to grabbing individual keys.

With the current code the keyboard gets ungrabbed, as far as I can tell,
only by the final XCloseDisplay() in cleanup(), as the XUngrabKey()
there effectively does nothing.