Find interpretation of modifier masks through the X server's modifier

Find interpretation of modifier masks through the X server's modifier
mapping and keyboard mapping by finding which modifier masks are
associated with keycodes mapped to the keysyms for modifier keys.  This
now follows the ICCCM, rather than assuming that Mod1 means Meta, Mod2
means Super, and so on.  Edwin also now recognizes Super and Hyper in
key presses as a result; before it would ignore any modifier masks
other than Control and Mod1 (Meta).