symbols: fix bug in modifier_map handling

symbols: fix bug in modifier_map handling

The code used to match a keysym to a keycode (see added comment)
differed in behavior from xkbcomp, always taking the first key it found.
This caused some incorrect interpretation of the xkeyboard-config data,
for example the one corrected in dump.data (see the diff): since the
de-neo layout sets the both_capslock option, the Left Shift key (LFSH)
has the Caps_Lock keysym in group 4 level 2; now since
    keycode(Left Shift) = 50 < keycode(Caps Lock) = 64
the Left Shift one was picked, instead of the Caps Lock one which is
group 1 level 1. The correct behavior is to pick according to group,
level, keycode.

Signed-off-by: Ran Benita <ran234@gmail.com>