place windows only in areas where is a physical display/monitor

[anthonykirby]

If a user has multiple physical displays, but there is no display at 0,0 , clusterssh windows appear overlapped.

e.g. a small display to the left of a larger one

'     +----+
+----+| D2 |
| D1 ||    |
+----++----+

Running cssh with --debug 3 you can see that it's trying to create windows around 0,0, i.e. in the area in the top left where there's no display, then (presumably) the window manager puts them all on top of each other, as near as it can on one of the physical displays.

It's possible to do a workaround by setting config screen_reserve_left=<width of D1>, but it would be better if clusterssh were aware of where each display is.

This is related to #131, #68

example output of xrandr --listmonitors:

Monitors: 2
 0: +*DP-1-1 2560/597x1440/336+1920+0  DP-1-1
 1: +eDP-1 1920/344x1080/194+0+715  eDP-1