[RP] quick fixes

[shawn]

* cmd_generate doesn't segfault when there is no current window

* you can split empty frames

* the wait-for-a-key icon uses a standard X cursor

* the out of sync X errors should be fixed because X commands aren't
  executed in signal functions. A HUP signal properly restarts ratpoison.

* The split keys have changed to C-t s, C-t C-s for "split" and C-t S, C-t C-S 
for "vsplit"