Use the KeyCode to directly find the modifier mask. (Fix bug 5173)

Use the KeyCode to directly find the modifier mask. (Fix bug 5173)

We were finding the KeySym first, and then converting back to a modifier mask.
But KeySym on a key's release can differ from on its press, and we don't need
them to determine the modmask from the keycode.

[setxkbmap -option "grp:shifts_toggle"] turns Shift_L into XK_ISO_Prev_Group on
key release, and Shift_R into XK_ISO_Next_Group.