Preliminary multiple monitor support. This works pretty well in rootless and full-screen Quartz mode; IOKit is still hardwired for one monitor.
* No GUI. "+xinerama" must be added to the command line. For efficiency, +xinerama should not be added if only one monitor is present.
* The cursor sometimes disappears during app switch.
* X Windows can't cope if the monitors are rearranged. Don't do that.
* X Windows prefers if the total screen region is still rectangular. Otherwise, windows and menus are sometimes placed partly or fully offscreen.
* The X11 main screen is the same as the Mac main screen. This can be changed fairly easily.