Fix restoring the kbd_mode part of the keyboard state in grab/ungrab.

Fix restoring the kbd_mode part of the keyboard state in grab/ungrab.
Simply change the mode to K_XLATE using a local variable and use the
grab level as a flag to tell screen switches not to change it again,
so that we don't need to switch scp->kbd_mode.  We did the latter,
but didn't have the complications to update the keyboard mode switch
for every screen switch.  sc->kbd_mode remains at its user setting
for all scp's and ungrabbing restores to it.