tmux: kludge a fix to erroneous cpu numbers in statusbar

previously we invoked vmstat and parsed its output to report on
supposedly 'recent' CPU consumption.  however, as invoked, vmstat
outputs cumulative-since-boot CPU numbers, as that's all the kernel
interface supplies.  having vmstat wait a few seconds to report
last-few-seconds numbers is possible, but is not recommended
(see tmux/tmux#797 (comment) )
and results in status line updates at rapid and irregular intervals
(see tmux-plugins/tmux-cpu#15 ).  this is
distracting, and also makes tmux create high load on an ancient raspi.

so instead, we kludge using tmux as a sort of background job manager,
launching vmstat on a high-numbered window and parsing its output
into a file underneath our systemd-created session tmpfs, from which
the tmux status line reads it.

vmstat is used over iostat/mpstat/etc as it should be always available
(at least on Debian systems where procps is Priority: important).

README.md

@@ -5,3 +5,4 @@
 
 * include a ssh/config broken into a common fragment and (when needed)
   per-machine fragments glued together using Include and a config.d dir
+  * problem: how to condition on origin machine hostname?

tmux/.tmux.conf

@@ -4,7 +4,10 @@ bind C-z send-prefix
 bind z last-window
 bind r source-file ~/.tmux.conf
 
+# absurd way to run a 'stats daemon' which is then used by the status line
+new-window -d -a -t 99 -n vmstat 'echo dm1zdGF0IC1TIG0gMSB8IHRlZSAvZGV2L3N0ZGVyciB8IGdyZXAgLVAgJ15bMC05WzpzcGFjZTpdXSskJyAtLWxpbmUtYnVmZmVyZWQgfCBhd2sgJ3twcmludCAxMDAtJDE1OyBmZmx1c2goKX0nIHwgKHdoaWxlIHJlYWQgRiA7IGRvIGVjaG8gIiRGIiA+ICRYREdfUlVOVElNRV9ESVIvY3B1IDsgZG9uZSkK | base64 -d | zsh'
+
 set-option -g status-style  "fg=cyan,bg=black"
 set-option -g window-status-current-style "fg=cyan,bold,bg=blue"
-set-option -g status-right "#(vmstat | tail -n1 | awk '{print 100-$15}')%%cpu #(free | grep Mem | awk '{printf \"%%.0f\", $4/$2*100}')%%mem #H #(date -u '+%%H:%%M %%Z')"
+set-option -g status-right "#(cat $XDG_RUNTIME_DIR/cpu)%%cpu #(free | grep Mem | awk '{printf \"%%.0f\", $4/$2*100}')%%mem #H #(date -u '+%%H:%%M %%Z')"
 set-option -g status-interval 10