Fallback on xrandr to get linux DPI info.

xdpyinfo is not always available on Linux (even with a DE), whereas
xrandr is (AFAICT) at least dependency of KDE and XFCE.  But xrandr
fails when used remotely, so keep it too.
I can live with that -- but can't we further limit the number of lines in the `try` portion by moving the `match... if match ...` lines into the `else` of the `try/except`, since those shouldn't fail?