sync with src changes
move validation of pointer Button into conf_mousebind so we check
validity during the parse phase and not bother adding it to the queue,
instead of each time a client needs to grab (when it's too late);
furthermore, make this a config error, stop parsing and load the
default config.
get rid of long standing XXX: now that we configure screens based on
config options, add the keybinding GrabKey calls here
since we don't have any screens yet, there's no reason to conf_{,un}grab
here; just build keybindingq.