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.