Xdefaults, xinitrc: dynamically calculate font size

Recent changes in OpenBSD made my laptops share different (althought
correctly calculated) DPIs. Add an script and rework X resources loading
so:

 1. load ~/.Xdefaults into xrdb for baseline
 2. run the script to compute a DPI and xterm's faceSize so it respects
    some values previously defined in this repo

As an upside, now XTerm.VT100.faceSize can be queried, so use that in
hlwm theme.

xdg-config-dir/herbstluftwm/theme

@@ -29,7 +29,9 @@
 : ${hlwm_foreground="#fff8f0"}
 
 : ${hlwm_font="-misc-fixed-medium-r-normal--14-130-75-75-c-70-iso10646-1"}
-#: ${hlwm_face="monospace:size=10.5"}
-: ${hlwm_face="monospace:size=8.0"}
+
+_hlwm_faceSize=$(xrdb -get XTerm.VT100.faceSize)
+: ${hlwm_face="monospace:size=${_hlwm_faceSize:-10.5}"}
+unset _hlwm_faceSize
 
 : ${hlwm_font_height=17}